Il tema della verità nell’Expositio Libri Peryermenias di Tommaso d’Aquino - Galluzzo

265AQUINAS ON NOMINAL AND REAL DEFINITION

GABRIELE GALLUZZO

To Grasp Something of the Thing Itself.Aquinas on Nominal and Real Definition

In this paper I wish to present Aquinass interpretation of a particularlydifficult section in the second book of Aristotles Posterior Analytics, i.e. Chs.8-10. The different issues I am going to touch upon all concern Aristotlesdistinction between nominal and real definition : Does Aristotle have a notionof nominal definition ? Do nominal definitions have existential import ? Andhow do they relate to scientific, real definitions and to scientific enquiry ingeneral ? I shall start off with a sketchy presentation of some problems inAristotles text and of some solutions that may be offered in response to them.I shall then move to Aquinass understanding of Aristotles text. In presentingAquinass solutions to the problems posed by Aristotle I shall also try to seewhich, if any, of the modern interpretations best fits Aquinass reading. Mygeneral view is that Aquinas provides a consistent explanation of Post. An., II,8-10, which is fundamentally in line with one of leading contemporaryinterpretations and in particular with the one proposed by J. Barnes. In theconcluding remarks, I shall spend a few words on the sense in which, inAquinass eyes, Aristotles distinctions in Post. An., Book II support his viewson existence and essence. I shall also try to make it clear that this furtherpoint of Aquinass goes much beyond Aristotles text and, to some extent, iseven against its spirit.

1. ISSUES IN POST. AN., II, 8-10

a) Knowledge of essence and knowledge of existence

As is well known, in Post. An., II, 1 Aristotle lists four kinds of question wemay ask about something and, consequently, four different pieces of knowledgewe come to acquire once each type of question is positively answered. Thus,we come to know : (1) that something is the case ; (2) why something is thecase ; (3) whether something exists ; (4) what something is. As the clause isthe case shows, (1) and (2) should be taken to concern states of affairs, whichcan be expressed in a proposition. In Aristotles ontology, states of affairs areanalysable in terms of a certain propertys belonging to a certain subject.

266 GABRIELE GALLUZZO

Therefore, to know (1) means to know that a certain property belongs to acertain subject, whilst to know (2) means to possess the explanation or thecause of a certain propertys belonging to a certain subject. Unlike knowledgeof types (1) and (2), knowledge of types (3) and (4) does not concern thebelonging of a property to a subject, but rather conveys information about thesubject itself. By (3) we come to know that a certain thing exists, whilst by (4)we come to possess the knowledge of what a thing is, i.e. the knowledge of theessence of such a thing. As Aristotles examples in Post. An., II, 1 and 2 makeclear, the object of knowledge of types (3) and (4) is not restricted to genuine,ontological subjects, i.e. substances, but includes everything that can be theobject of a scientific enquiry, i.e. everything that can figure as a subject in theconclusion of a demonstration. Eclipses, for instance, or thunders are notgenuine subjects, i.e. substances, from an ontological point of view, butrather events and hence, presumably, accidents. However, they are appropriateobjects of scientific knowledge, in so so far as they are things about which wecan ask whether they exist and what they are, and of which we can prove thatthey possess certain characteristic properties.

My main concern here is the relation between knowledge of type (3) andknowledge of type (4), i.e. the knowledge of the existence of a thing and theknowledge of its essence. In Post. An., II, 1, 89b34 Aristotle says that, havingcome to know that a certain thing exists we seek what it is. This suggests thatour knowledge of the existence of a thing precedes our knowledge of itsessence. In other words, we can start enquiring into what a thing is only afterwe have ascertained that it exists. That the knowledge of a things existenceis prior to the knowledge of its essence is explicitly affirmed by Aristotle inCh. 81 , where he makes also clear the relation of priority between knowledgeof types (1) and (2). In general, Aristotle says, we enquire into why somethingis the case, i.e. why S is P (type (2)) only after knowing that something is thecase, i.e. that S is P (type (1)) : even though, occasionally, we may come toknow that something is the case and why something is the case at the sametime, it is impossible for us to know why something is the case before we knowthat it is actually the case. Analogously, in the case of knowledge of types (3)and (4) it is impossible for us to know what a thing is, i.e. to know the essenceof a thing, before we know that it exists. Thus, Aristotles general doctrine isclearly that, standardly, our knowledge of somethings existence precedes ourknowledge of somethings essence.

Ch. 8 itself, however, complicates this picture in several directions. Thefirst thing to note is that, when Aristotle talks about the knowledge of the

1 Cf. also ARISTOTLE, Post. An., II, 8, 93a16-20. Cf. also II, 7, 92b4-5.


essence of a thing, he must clearly have in mind a full and complete accountof what a thing is, i.e. a scientific definition revealing the full essence of theobject we are investigating into. We shall see later on what a scientificdefinition looks like and how it differs from other, non-scientific accounts ofa thing. However, as early as in Ch. 2 Aristotle gives us some indications as toone distinguishing feature of a scientific definition. Part of his argument inCh. 2 consists in showing that also the knowledge of the essence of a thing isan instance of knowing the cause or the explanation of something (thus,knowledge of type (2) and knowledge of type (4) turn out to be closelyconnected, if not in some sense identical). In all likelihood, Aristotles pointis that the essence of a thing x is the explanation or cause of xs existence, atleast in the sense that the essence of x explains why x exists in the way it does,i.e. why it possesses certain fundamental features that characterise its beingthe kind of object it is. Therefore, a scientific definition of x must include thecause or explanation of xs existence, if not straightaway identify with astatement of such a cause or explanation. Thus, Aristotles particularlydemanding understanding of scientific definition leaves room for there to beother, partial accounts of what a thing is. Such accounts clearly fall short ofa complete, scientific definition, but may still be useful in the preliminarystages of a scientific enquiry, i.e. in the stages before we arrive at a fullscientific understanding and definition of the thing we are after.

This distinction among different accounts of what a thing is links up witha general difficulty concerning the relation between our knowledge of athings existence and our knowledge of a things essence. For if we cannotknow what a thing is without knowing in advance that the thing exists, howare we to acquire the knowledge that a thing exists ? Now, Aristotle is clearlynot of the opinion that we can come to know that a thing exists without havingat least some account of the kind of thing whose existence we are trying toknow. If we did not possess any account of the thing whose existence we setourselves to establish, we would not be able to identify the thing in questionshould we bump into it, nor, a fortiori, to establish its existence. For theexistence of a thing is always the existence of a certain kind of thing.Therefore, it seems that, in order to know that a thing exists, we need someaccount of the thing whose existence we are trying to know. Such an account,however, cannot be the scientific definition of the thing, i.e. the definitionwhich fully expresses the essence of the thing, because the knowledge of sucha definition is posterior to our knowledge of the existence of the thing. Thus,what is the account that enables us to establish the existence of a thing, if itis not its full definition ? Which features or properties of the thing does itcontain, if it does not contain all its essential features ? Aristotle tackles thesedifficulties in the course of Ch. 8s argument and comes back to them in Ch.


10, where he lists and describes different kinds of definition or account. Thequestion of the nature of Aristotles nominal definition mainly revolvesaround understanding what Aristotle says in Ch. 8 and how it connects withthe discussion of the different kinds of definition in Ch. 10. Let me start,therefore, with the question of how we come to know the existence of a thing,which Aristotle discusses in Ch. 8. In the next section, I shall move toAristotles classification of the different kinds of definition in Ch. 10.

In Post. An., II, 8, 93a21 ff. Aristotle says that we can come to know theexistence of a thing in two ways. At times, we come to know the existence ofa thing only accidentally, at others by grasping something of the thingitself : for instance by grasping that thunder is a certain noise in the clouds,that eclipse is a certain privation of light, that man is a certain animal andthat soul is something moving itself. Aristotle further remarks (93a24-29) thatwhen we come to know the existence of a thing only accidentally, we have nograsp of what the thing is, i.e. of its essence, for in a sense we do not even knowthat the thing in question exists. When, on the contrary, we come to know thata thing exists by grasping something of the thing, we are on our way toknowing what the thing is, i.e. to knowing the essence of the thing. It seemsto me that, since Aristotle at 93a29 equates to grasp something of the thingitself with to grasp something of what a thing is , i.e. something of itsessence, his distinction between two ways of knowing the existence of a thingis most naturally read in the light of the distinction between accidental andessential properties. In other words, we come to know the existence of a thingonly accidentally when we know that a thing exists through an accountindicating only accidental features of the thing we are after. In this case, thereis a sense in which we cannot even be said to know that a thing exists, becauseto know the accidental features of a thing is not to know the thing as the kindof the thing it is. Moreover, everything that can be properly said to be a thingpossesses a certain kind of non-accidental unity, which is reflected in theunity of the account saying what the thing is. And an account which pointsonly to accidental features of a thing does not display the kind of unityrequired, in that the different accidental features of a thing do not form withone another a kind unity comparable to the unity that a proper object ofdefinition is supposed to possess. When, by contrast, we know the existenceof a thing through grasping something of the thing, we possess an accountwhich indicates some of its essential features. Such an account still falls shortof a proper scientific definition of the thing, in that it mentions only some ofits essential features and only part of its essence. However, it can contributemuch towards forming a full, scientific definition of the thing we areinvestigating into. Such a reading of Aristotles distinction between two ways


of knowing the existence of a thing has been advocated by Barnes in particularand seems to me the most natural way of taking the Greek text2 .

How does the account of Post. An., II, 8 I have just provided relate to thetopic of nominal definition ? On this account, a definition which makes knownonly some accidental features of a thing and, consequently, provides us onlywith an accidental knowledge of its existence counts as one instance of nominaldefinition. The nominal definition of the things which we take to be objects ofscientific investigation, things such as thunders and eclipses, is an accountthat explains the meaning of the name of the thing in question by pointing tosome of its accidental and superficial characteritics. Thus, on the view I havejust proposed the nominal definition of the objects of scientific investigationshould be sharply distinguished from the definition which grasps something ofthe thing itself : although undoubtedly partial, the latter is an account whichcontains one or more elements in the essence of the thing, whilst a nominaldefinition only explains the meaning of the name of a thing by indicating someof its accidental features. However, according to this reconstruction of Aristotlesdoctrine of definition, the accidental accounts of Post. An., II, 8 are not the onlyinstances of nominal definition that there are. Nothing of what Aristotle saysrules it out that there might be nominal definitions of things that do not exist,of things, for instance, such as goatstags. In Post. An., II, 7 Aristotle contraststhe definition revealing what a thing is with an account that merely explainsthe meaning of a things name3 . He also remarks that there is no definition ofwhat a non-existent thing is, but only an account of what its name means. Thus,in the case of non-existent things the nominal definition will be an account ofwhat the name of such things signifies, an account which does not indicate anyreal features of the things defined, simply because the things in question do notexist and so possess no real features for us to indicate. Of course, the nominaldefinitions we are most interested in are those concerning existing objects, butthere does not seem to be any particular reason to deny non-existing objects anominal definition, given that, also in the case of existing objects, a nominaldefinition only points to their accidental features. In other words, for supportersof this account, nominal definitions have no existential import, i.e. they are notconfined to things that exist, but rather range over all things that possess ameaningful name.

To sum up, according to this account of Aristotles doctrine of definitionin the Posterior Analytics, we would have three different kinds of definition :(i) the nominal definition of a thing, i.e. an account explaining the meaning

2 Cf. J. BARNES, Aristotle. Posterior Analytics, Clarendon Press, Oxford 19932, pp. 218-219.3 Cf. ARISTOTLE, Post. An., II, 7, 92b4-8 ; 26-34.


of a things name, be it an existent or a non-existent thing. In the case of anexistent thing, a nominal definition mentions some accidental features of thething in question and provides us with a merely accidental knowledge of itsexistence ; (ii) a partial definition of the thing, which grasps one or moreelement in the essence of the thing and so directs us towards the fullknowledge of the essence ; (iii) the complete, scientific definition of a thing,which is or at least contains a statement of the explanation of the thingsexistence. Let me call the present account Account A .

There are, however, other interpretations of Aristotles doctrine of definitionin the Posterior Analytics. One influential account, for instance, which hasbeen advanced some years ago by Bolton, does not differ from Barness somuch on the literal reconstruction of Aristotles distinctions in Post. An., II,8 as on the scope and general significance of nominal definitions4 . Accordingto Bolton, nominal definitions do have existential import, i.e. they areconfined to things that exist. Things that do not exist, things such as goatstags,may be given an account of what their name signifies, but such an accountdoes not count as a nominal definition, at least as Aristotle understands thisnotion. For a nominal definition is always an account of what something is,and things that do not exist are simply nothing. Boltons account is muchinfluenced by his reading of Ch. 10, to which I shall turn in the next section.Since now, however, we can see how this account applies to Aristotlesdistinction in Ch. 8. For Bolton, a nominal definition is an account whichenables us to have a grasp of the existence of a certain existing thing bypointing to some feature or other the thing itself possesses. Thus, all theaccounts Aristotle mentions in Ch. 8 must be kinds of nominal definition. Inother words, both the account by means of which we come to know theexistence of a thing only accidentally and the account by means of which weknow the existence of a thing by grasping something of the thing itself areinstances of nominal definition. The former account only mentions accidentalfeatures of the thing we are investigating into and so is of little use inscientific investigation. It may help us to fix the reference of a certain generalterm and to single out some uncontroversial specimens of a certain naturalkind or phenomenon, but it contributes little or nothing towards ourunderstanding of the essence of the kind or of the phenomenon. The latterkind of definition, by contrast, in so far as it grasps something of the thingitself, already directs us towards a full understanding of its essence. Anotheraspect of Boltons reconstruction is that he considers Ch. 8s examples of

4 R. BOLTON, Essentialism and Semantic Theory in Aristotle : Posterior Analytics, II, 7-10, The Philosophical Review , 85, 1976, pp. 514-544.


nominal definition as mere examples, which do not exhaust the scope ofnominal definition. In particular Bolton suggests we should admit ofanother kind of nominal definition in addition to the ones explicitly mentionedin Ch. 8, i.e. the one that points to some per se accidents of the thing defined.It is somehow intermediate between the two kinds of nominal definition I havejust described : in so far it points to per se accidents, it differs from the nominaldefinition that makes known the existence of a thing only accidentally ; on theother hand, in so far as it points to some kind of accidents, although per se ones,it also differs from the definition that grasps something of the thing itself.However, it can be of some use in scientific inquiry, in that it reveals at leastsufficient conditions for a certain thing to exist, even if such conditions areultimately explained by the deeper, essential properties of the thing in question.Boltons account strikes me as textually difficult. In particular, I fail to seewhere and how Aristotle would bring into the picture the third kind ofdefinition Bolton alludes to. However, also this second account may be usefulin understanding Aquinass position. Therefore, let us keep it in mind and callit Account B . The distinguishing feature of Account B is that it takes all theexamples of definition in Ch. 8 and in particular also the definition thatgrasps something of the thing itself as instances of nominal definition.Moreover, Account B suggests that all kinds of nominal definition must have,for Aristotle, existential import, i.e. are confined to things that exist.

Among the different interpretations of Post. An., II, 8 which have beenadvanced over time, I would like to mention another one, which is to someextent a variation on and a refinement of Boltons view. The account, whichI shall label Account C , has been strongly defended by Demoss andDevereux5 . Demoss and Devereux agree with Bolton that nominal definitionsdo have existential import and so are confined to things that exist. However,they introduce a even more restrictive and metaphysically loaded notion ofwhat a nominal definition is supposed to be. According to them, the accountof a thing which only mentions accidental features of the thing itself, and sogives us only an accidental knowledge of its existence, does not count as anominal definition. An account of eclipse, for instance, which only mentionsaccidental features of the eclipse may still serve in some sense as an accountof what the name eclipse means, but it does not count as a nominal definitionof the eclipse in the Aristotelian sense of the term. For the task of a nominaldefinition is precisely to help us to acquire a proper, i.e. non-accidental,

5 D. DEMOSS, D. DEVEREUX, Essence, Existence and Nominal Definition in Aristotles PosteriorAnalytics, II, 8, Phronesis , 33, 1988, pp. 133-154. Account B is foreshadowed by R. SORABJI,Definitions : Why Necessary and in What Way ? in E. BERTI ed., Aristotle on Science. The PosteriorAnalytics , Antenore, Padova 1981, pp. 205-244.


knowledge of the existence of the thing, and Aristotle is explicit in saying thatan account which mentions only accidental features provides us with a merelyaccidental knowledge of a things existence. Thus, on this view, the mostchracteristic examples of nominal definition are the definitions that graspsomething of the thing itself, i.e. the partial definitions of a things essence6 .They differ from scientific definitions in that they do no mention the cause orexplanation of the things existence, even if they direct us towards such acause and explanation. Note that Account C is exactly the reverse of AccountA, at least in so far as the nominal definitions of the objects of scientificinvestigation are concerned : on Barness view the nominal definitions of suchobjects are precisely those definitions that mention only accidental featuresof the thing defined, whilst on Demoss-Devereuxs view accidental accountsdo not count as nominal definitions. What is more, Boltons and Demoss-Devereuxs notions of nominal definition are quite different from the modernone. Far from confining themselves to giving the meaning of a term, nominaldefinitions point towards real features of the thing defined. The fact that theyalso have existential import means that definitions that only spells out themeaning of a name, the meaning, for instance, of the name of a non-existingthing, are not even nominal definitions. Barness view on nominal definitions,by contrast, is closer to the moder one, in that, according to him, the maintask of a nominal definition is precisely to explain the meaning of a thingsname, be the thing in question existent or non-existent. However, at least inthe case of existing things, nominal definitions do more than that and pointto some real, though merely accidental, features of the thing defined.

6 As we shall see later on (cf. infra, p. 276), for Demoss and Devereux nominal definitions arenot confined to things like thunders and ecplises, but also concern other kinds of objects. In Post.An., Ch. 9, Aristotle distinguishes between things that have a cause other than themselves andthings that do not have a cause other than themselves. Aristotles distinction is not entirely clearbut one reasonable suggestion is that things that have a cause other than themselves are the itemsthat can appear as subjects in the conclusion of a syllogism, whereas things that do not have acause other than themselves are the primitive items in a certain scientific branch (for instance theunit in the case of arithmetic) and so do not appear in the conclusion of syllogisms. Accordingly,in Post. An., Ch. 10 Aristotle holds that the definition of things like thunder and eclipse can be theconclusion of a syllogism. As the text makes clear, the definition of thunder and eclipse thatfigures in the conclusion of a syllogism is not their scientific or complete definition, but rather thepartial definition of their essence, which according to Demoss and Devereux is one instance ofnominal definition. The definition of the primitive terms of a scientific branch, i.e. of things suchas the unit, cannot appear, by contrast, in the conclusion of a syllogism and must simply beassumed. On Demoss and Devereuxs interpretation, also the definition of the primitive terms ofa science counts as one instance of nominal definition. Thus, nominal definition covers both thederivative and the primitive terms of a certain scientific branch. In both cases, nominal definitionseems to have existential import. Moreover, it is clear that Ch. 8 does not deal with primitive termsbut rather with derivative ones, i.e. things such as thunders and eclipses.


The accounts I have just sketched out by no means exhaust the differentinterpretations of Post. An., II, 8-10 scholars have been proposing over time,especially when points of detail are considered7 . However, they shouldprovide us with a general grid to evaluate Aquinass interpretation. Beforeturning to Aquinas, however, I shall test out the different accounts againstAristotles explicit classification of the different kinds of definition in Ch. 10.

b) Kinds of definition

In Post. An., II, 10 Aristotle lists different kinds of definition. There is noconsensus among scholars as to how many kinds are actually listed and as tohow Aristotle thinks they relate to each other. I shall start, therefore, bysimply reporting Aristotles different statements on definition ; then I shallillustrate some of the difficulties his classification presents.

Df1) Since definition is said to be an account of what something is, onekind of definition will be an account of what its name, or some other name-like expression, means.

Df2) Another kind of definition is an account that shows why somethingexists. Such a definition will be like a demonstration of what something is,differing from it only in arrangement.

Df3) A further type of definition is the conclusion of the demonstration ofwhat something is, e.g. thunder is a noise in the clouds.

Df4) The definition of an immediate term is an indemostrable positing ofwhat it is.

Aristotles classification is particularly difficult and has always baffledinterpreters. Before seeing how the different accounts I have listed in theprevious section make sense of it, let me lay down some facts about whichthere is a certain, relative agreement. I have described Df4) as the definitionof an immediate term. In Post. An., II, 9 Aristotle distinguishes betweenthings for which there is something else which is their explanation, i.e. thingswhich have a cause other than themselves, and things for which there is not

7 For a more recent reconsideraton of the issues raised in Post. An., II, 8-10 see : D. CHARLES,Aristotle on Meaning and Essence, Oxford University Press, Oxford 2000. Charless analysis is setagainst the background of the contemporary debate about essentialism.


something else which is their explanation, i.e. things which do not have a causeother than themselves. Aristotles distinction is variously interpreted. On onereasonable interpretation, the intended distinction is between the primitiveterms of a science, i.e. the items that do not appear as subjects in the conclusionof a syllogism, and the derivative ones, which, by contrast, appear in theconclusions of syllogisms8 . On any account, however, the things which do nothave an explanation other than themselves are the things whose definition isDf4), i.e. the indemostrable positing of what they are. Whether such thingsare the primitive, immediate terms of a science or something else, they areclearly not the kind of things Aristotle talks about in Ch. 8, where reference isclearly made to things which have an explanation other than themselves, thingsfor instance like thunders and eclipses. Therefore, we can leave aside Df4) forthe time being and concentrate on the other three kinds of definition.

Also the relation between Df2) and Df3) is relatively clear. Df2) is thecomplete, scientific definition of a thing. It provides a complete account ofthe essence of a thing by mentioning the cause or explanation of its existence9 .Aristotle suggests that we can obtain such a definition by re-arranging theterms of a special kind of demonstration, that is the demonstration of whatthe thing is. What Aristotle means, of course, is not that we can come todiscover the cause of the existence of a certain thing by building up a specialkind of syllogism, but rather that, once we have discoverd the cause orexplanation of a certain thing or phenomenon, we can build up a syllogismwhere such a cause figures as the middle term. And then by re-arranging theterms of the syllogism in question, we can obtain a full, scientific definitionof the thing. Aristotles example of Df2) is : Thunder is a sound of fire beingquenced in the clouds , which presumably is equivalent to : Thunder is anoise in the clouds caused by the quencing of fire . Df3) is precisely theconclusion of the syllogism we are able to build up once the cause of a thingis discovered. According to Aristotle such a conclusion counts as one kind ofdefinition of the thing. He also provides an example of Df3), i.e. thunder is

8 For this interpretation see : BARNES, Aristotle cit., pp. 221-222 ; Cf. DEMOSS, DEVEREUX,Essence, Existence cit., pp. 134-138. For a different reading, accordind to which Ch. 9sdistinction should be read in the light of the opposition between events/accidents and substances,see : W. D. ROSS, Aristotle. Prior and Posterior Analytics, Clarendon Press, Oxford, 1949, p. 633.

9 In Post. An., II, 2-9 (and especially in II, 2) Aristotle expresses himself as though thescientific definition of a thing were simply identical with a statement of the explanation or causeof its existence. From II, 10, however, it clearly emerges that what he means is that the scientificdefinition contains a reference to the explanation of a things existence. It should not beforgotten (i) that II, 1-2 give only the general structure of scientific investigation by indicatingthe general questions scientists are supposed to answer and their mutual relations ; (ii) that thediscussion in II, 3-7 is deliberately aporematic.


a noise in the clouds . It should not pass unnoticed that the example Aristotleoffers is precisely one of the examples in Ch. 8 of an account that graspssomething of the thing itself. Thus, the kind of syllogism Aristotle seems tohave in mind is something along the following lines :

(1) Noise in the clouds holds of the quencing of fire(2) The quencing of fire holds of thunder(3) Noise in the clouds holds of thunder

Aristotles syllogism can be and as a matter of fact has been reconstructed differently, and there are also some other problematic pointsof detail10 . But, on a whole, the relation between Df2) and Df3) seems to besufficiently clear. Df3) is the partial account of a things essence, whilst Df2)is the full account of the essence : Df2) is obtained by re-arranging the termof the syllogism which has Df3) as its conclusion.

Much of the dispute concerns Df1), Aristotles nominal definition, and itsrelation to the other kinds of definition. One major problem here is whetherDf1) is another kind of definition, i.e. a kind of definition over and above Df2),Df3), Df4), or it should be identified with one of the aforementioned ones.That the letter is a concrete possibility seems to be suggested by Aristotlesrsum of the different kinds of definition in II, 10, 94a11-14, where he onlymentions Df2), Df3) and Df4) and makes no reference at all to Df1). Thus,some interpreters have thought that we are in fact confronted with only threekinds of definition and hence Df1) should be identified with one of those threetypes. Ross, for instance, holds that we should simply identify Df1), Aristotlesnominal definition, with Df3)11 . Aristotles example of Df3) in fact is thunderis a noise in the clouds, which is also one of the examples in Ch. 8 of anaccount which grasps something of the thing itself. Rosss suggestion iscongenial to Account C, according to which nominal definitions are preciselyaccounts which mention some elements in the essence of a thing and so directus towards a full understanding of its essence. Now, Df1), nominal definition,is defined by Aristotle as an account of what something is . This suggeststhat Df3), the partial account of a things essence must be an instance ofnominal definition. For Ross, therefore, the reason why Df1) is not mentionedin Aristotles final list at 94a11-14 is simply that Df1) is nothing but Df3).

10 For the interpretation I suggested in the text see : BARNES, Aristotle cit., p. 224. For acritical discussion of the interpretative options available see : M. MIGNUCCI, Aristotele. AnaliticiSecondi, Organon IV, Laterza, Bari 2007, pp. 276-278.

11 Cf. ROSS, Aristotle. Prior and Posterior cit., pp. 634-636.


As a matter of fact, supporters of Account C agree with Ross that Df3) isan instance of nominal definition12 . Demoss and Devereux also agree in somesense that Aristotle presents in Ch. 10 three types of definition. What theyreject is Rosss unqualified identification of Df3) with Df1)13 . For one of theconsequences of Rosss view is that Df3) is the only kind of nominal definitionthere is. However, it is clear that the objects of which Df3) is a definition arethose objects that have a cause or explanation other themselves, i.e. thingssuch thunders and eclipses. But Aristotle also mentions objects that do nothave a cause or explanation other than themselves, i.e. the objects whosedefinition is Df4). And Demoss and Devereux insist that also Df4) is aninstance of nominal definition. The main difference between Df3) and Df4),i.e. between the two types of nominal definition Aristotle would admit of, isthat the nominal definitions of objects that have a cause other than themselves,Df3), figure as conclusions of a syllogism, whilst the definition of things thatdo not have a cause other than themselves cannot figure as conclusions of asyllogism. Thus, on Account C, Df1) does not point to one separate kind ofdefinition, but rather counts as a general characterisation of nominaldefinition, which includes both Df3) and Df4) and is naturally contrasted withreal definition, i.e. an account that, like Df2), reveals the cause or theexplanation. In other words, Aristotle would first distinguish between nominaldefinitions and real definitions and then further specify that there are twodifferent kinds of nominal definition, i.e. Df3) and Df4).

There are difficulties with this way of interpreting Aristotles distinctionsand in particular with considering Df3) as an instance of nominal definition.At 93b29-36, when introducing Df1), Aristotle says :

Since a definition is said to be an account of what something is, it is clear onetype will be an account of what its name, or some other name-like account,means e.g. what triangle means. When we grasp that this exists, we seek whyit is so. But it is difficult to take anything in this way if we do not know that itexists. The explanation of the difficulty was given earlier : we do not even knowwhether it exists or not, except accidentally 14 .

If we take in this way at 93b33 to mean by means of a nominaldefinition , i.e. Df1), it becomes virtually impossible to take Df3) as aninstance of nominal definition, as supporters of Account C think we should,

12 Cf. DEMOSS, DEVEREUX, Essence, Existence cit., pp. 134-138.13 Cf. ibid., pp. 136-137 (esp. Footnote 10).14 Cf. ARISTOTLE, Post. An., II, 10, 93b29-36 (Barness traslation). The emphasis is mine.


or simply identify nominal definitions with Df3), i.e. with partial definitionsof the essence, as Ross would have it. For, when read in this way, Aristotlespassage comes to say that Df1), i.e. nominal definition, is the kind of accountin virtue of which we come to know the existence of a thing only accidentally,and not the account in virtue of which we grasp something of the thing itself.In other words, the text seems to support an intepretation along the lines ofAccount A, i.e. Barness interpretation15 . Nominal definitions explain themeaning of the name of things. In the case of things that exist, they also pointto some accidental features of things and so enable us to grasp the existenceof such things only accidentally. In a way, when we have only a nominaldefinition of a thing, we cannot even be said to really know that the thing inquestion exists. This reading of the text is supported by Aristotles explicitreference, at 93b33-35, to Ch. 8s distinction between two ways of knowing theexistence of a thing, i.e. accidentally and by grasping something of the thing.If the expression in this way is read as I have suggested, Df1), nominaldefinition, goes with the account that makes us know the existence of a thingonly accidentally. On Account A, therefore, we have four kinds of definitionin Post. An., II, 10. The reason why Df1), i.e. nominal definition, is notmentioned in Aristotles final list is simply that it is scienfically uninteresting,in that it does not even provide us with a proper knowledge of the existenceof the thing we are after.

Of course, as always happens, supporters of Account C are not withoutsolutions to this difficulty. One might, for instance, read in this way differently and connects it with the mention, in the immediately precedinglines, of the knowledge of the explanation of a thing16 . In other words, onemight take Aristotles passage to mean that it is difficult to know a thing inthis way , i.e. to know the explanation of its existence, without knowing thatit exists. And when we have only accidental knowledge of a things existence, wecannot properly be said to know that it exists. On this reading, the mention ofthe accidental knowledge of a things existence has nothing to do with Df1), i.e.nominal definition, but is just a quick reminder of Ch. 8s doctrine, intended toexclude accidental accounts from the kinds of account Ch. 10s discussion isinterested in. Thus, Df1) can be still associated with the account which graspssomething of the thing itself and hence it becomes possible to take Df3) as aninstance of nominal definition. This interpretation of the Greek text is perfectlypossible. However, Aristotles reference to accidental knowledge in the nearby,as it were, of Df1) is somehow disturbing for supporters of Account C.

15 Cf. BARNES, Aristotle. Posterior Analytics cit., p. 223.16 Cf. DEMOSS, DEVEREUX, Essence, Existence cit., p. 148.


What about account B), Boltons view ? Bolton sticks to the traditionalidea that Ch. 10 presents four kinds of definition. He also reads the section onDf1) in line with Account A and so connects Df1) with the account whichmentions only accidental features of the thing defined. Bolton insists, however,that nominal definition is not confined to Df1) and hence both Df1) and Df3)introduce kinds of nominal definition. Df1) is a nominal definition whichmentions only accidental features of the thing defined. Scientifically, it is theleast interesting kind of nominal definition and somehow slips from Aristotlesfinal list at 94a11-14. Df3), by contrast, is described as the conclusion of thedemonstration of what something is. Thus, Df3) must include only the kindsof nominal definition which can figure as conclusions of a syllogism. In otherwords, Df3) includes the other two kinds of nominal definition Boltonattributes to Aristotle : the nominal definition which gives part of the essenceof a thing and that which mentions some of its per se accidents. For per seaccidents too usually figure in the conclusions of syllogisms. I must confessI find Account Bs reading of Ch. 10 the most difficult to defend. For the textdoes not seem to suggest that there is any other nominal definition apart fromDf1). Thus, we seem to be left with two alternatives : either to take Df1) asAristotles nominal definition and contrast it with Df3), i.e. the definitioncontaining something of the essence of a thing, or to take Df1) as Aristotlesnominal definition and identify it with Df3). Account B wants to have it bothways, but the text does not support this reading.

II. AQUINASS READING OF POSTERIOR ANALYTICS, II, 8-10

a) The general picture

Aquinas presents a thoroughly consistent interpretation of Aristotlesdoctrine of definition in the second book of Posterior Analytics. As I shall try toshow in the following, his interpretation should be read along the lines of whatI have labelled Account A . In particular, Aquinas makes four points, whichare characteristic of Account A. (i) He interprets Aristotles distinction in II, 8between two ways of knowing the existence of a thing as a distinction betweenan account which points only to accidental features of the thing and an accountwhich grasps something of the things essence. (ii) He recognises in Aristotlesclassification in II, 10 four kinds of definition, i.e. Df1), Df2), Df3) and Df4),which I have described above. Actually, Aquinas reserves the title of definitiononly for Df2), Df3) and Df4), whilst he describes Df1) as a formula or accountwhich does not meet the strictest requirements for something to be a definition.Aquinass terminological choice is due to his taking a definition to signify in


some way or other the essence of the thing defined. As we shall see, Df1), onAquinass interpretation as well as on Account A, does not meet such arequirement. (iii) The Dominican Master clearly identifies Df3) with Ch. 8ssecond kind of formula, i.e. with the formula which grasps something of theessence of a thing. Such a formula is the one that can figure as the conclusionof a syllogism. (iv) The fourth point concerns nominal definition and may bethought to be more controversial. As a matter of fact, Aquinas does notstraightaway identify nominal definitions with Ch 8s first kind of account, i.e.the account which only points to the accidental features of a thing, nor does heidentify Df1) with nominal definition. On the contrary, in his commentary onPost. An., II, 10, he considers Df1) as a large class including both nominaldefinitions and accounts showing only the accidental features of a thing, thatis including all the accounts that does not signify in any way the essence of athing. Thus, it might seem that Aquinas in fact distinguishes nominal definitionsfrom accidental accounts. My analysis, however, will try to show that this is notactually the case. For in his commentary on Ch. 10, Aquinas starts off with avery strict understanding of nominal definition, according to which a nominaldefinition is an account which explains only the meaning of a certain namewithout mentioning any real characteristic of the thing corresponding to thename. In other words, Aquinas admits of the possibility of there being anaccount which only explains the meaning of a term without mentioning anyreal feature of the thing defined something very close to the modern notionof nominal definition. Such a notion of nominal definition fits in particularlywell with the case of non-existent objects : since non-existent objects are simplynot there, they do not present any real characteristics, be they accidental oressential, to be mentioned in an account. Aquinass strict sense of nominaldefinition, however, fits also the case of those items whose degree of unity is solow that they cannot be classified as things, properly speaking : an account ofthe Trojan War can certainly explain the meaning of its name, e.g. Ylias, butit would be hard to say that the Trojan War is a thing to which we can attributeessential or accidental properties in the same sense as we attribute essential oraccidental properties to an eclipse. Aquinass strict notion of nominal definitiondoes not do, by contrast, for the case of things that exist and also have asufficient degree of unity, i.e. things such as thunders and eclipses. For it seemsthat the definition which explains the meaning of the name of such things mustmention at least some of their real characteristics, however external andsuperficial they may turn out to be. Thus, the nominal definition of things thatare existent and have a sufficient degree of unity is after all an account whichmentions some of their accidental features. Aquinas, therefore, does not wantto contrast accidental accounts with nominal definitions taken generally, butsimply accidental accounts with nominal definitions in the stricter sense of


the term, i.e. those nominal definitions which do not mention any realcharacteristics of the thing defined. This, however, does not rule it out thataccidental accounts as well can count as nominal definitions. All thingsconsidered, on the issue of nominal definition as well, Aquinass interpretationis not very different from Account A. For on Account A too, nominal definitionsare not confined to existent objects, nor are they restricted to things thatpossess a sufficiently high degree of unity.

In the following, I am going to flesh out each of the foregoing claims. Letme start with Aquinass reading of II, 8.

b) Two ways of knowing existence

In his commentary on Post. An., II, 8, 93a14 ff., Aquinas starts fromAristotles claim that it is impossible to know the essence of a thing withoutknowing in advance its existence17 . However, he rightly takes the essence to mean the full essence of a thing, i.e. the essence which is captured in a full,complete definition18 . Thus, Aristotles words leave room for there to be someaccount of a thing, which falls short of a full scientific definition of itsessence, but still enables us to come to know its existence. As a matter of fact,later on in his commentary, Aquinas remarks that it is impossible to come toknow the existence of a thing without any previous account of what it is19 . Aswe have seen, Aristotle distinguishes in Ch. 8 two different ways of knowingthe existence of a thing, i.e. two different accounts of a thing which fall shortof scientific definition but may enable us to grasp the existence of a thing.Aquinass interpretation of Aristotles distinction is in keeping with AccountA, as can be seen simply by reading through his remarks on the text :

And [Aristotle] says that we can know that a thing exists without perfectlyknowing what it is, in two ways. In one way, in so far as we know someaccident of it, as in the case in which from the speed of the movement wejudge that there is a hare around. In another way, by knowing something ofthe essence of the thing (...) 20 .

17 Cf. SANCTI THOMAE DE AQUINO Expositio Libri Posteriorum, Lib. II, lect. 7, Opera Omnia, t.I*2, Cura et studio fratrum Praedicatorum, Commissio Leonina-Vrin, Roma-Paris 1989, p. 199,lin. 109-124.

18 Cf. ibid., p. 199, lin. 126-127.19 Cf. ibid., Lib. II, lect. 8, p. 203, lin. 92-99.20 Cf. ibid., Lib. II, lect. 7, p. 199, lin. 126-131 : Et dicit quod aliquam rem esse possumus

scire absque eo quod sciamus perfecte quid est, dupliciter : uno modo secundum quod cognoscimusaliquod accidens eius, puta si per velocitatem motus estimemus leporem esse ; alio modo per hocquod cognoscimus aliquid de essentia eius (...) . The translation is mine.


According to Aquinas, therefore, Aristotle distinguishes two different waysof knowing the existence of a thing. We can come to know a things existence bymeans of an account which only mentions an accident of the thing ; or we cancome to know a things existence through an account which grasps something ofthing itself, which Aquinas interprets as an account which mentions some (butnot all) of the elements in the very essence of the thing. The first kind ofknowledge of existence is merely accidental knowledge and does not contributeanything towards understanding the full essence of a thing and acquiring a full,scientific definition of it21 . In a way, when we know the existence of a thing onlyaccidentally, we cannot even be said to really know its existence. The wayAquinas explains this latter remark of Aristotles is interesting. He observes thatwhen we know only an accident of a thing we do not really know the thing itself,but something else. For accidents, though being truly predicated of a thing, arenot the thing of which they are predicated22 . Thus, Aquinas takes accidentalpredication to be an ontological relation obtaining between two really, i.e. mind-independently distinct things, whilst presumably essential predication obtainsbetween things that are not really, i.e. mind-independently distinct. The subjectand predicate of an essential predication express one and the same thing, thoughin different ways. The second kind of knowledge of existence consists in knowingthe existence of a thing by grasping something of its essence. In this case, wetruly know the existence of the thing and so are in a position to seek its fullessence and its scientific definition23 . Aquinas regards the examples Aristotleprovides in 93a21-24 ( Thunder is a sound in the clouds , Eclipse is aprivation of light , Man is an animal , Soul is a self-moving thing ) asexamples of definitions which mention some essential features of the thingdefined, i.e. as examples of partial definition of a things essence24 .

In conclusion, Aquinass interpretation of Ch. 8 perfectly squares withAccount A. The distinction Aristotle has in mind in the chapter is that betweenan account which mentions only some accidental features of a thing and apartial account of a things essence, i.e. an account which mentions some butnot all of the essential features of a thing.

c) Aquinass interpretation of Post. An., II, 10

According to Aquinas, in Post. An., II, 10 Aristotle admits of four differentaccounts of a thing, which perfectly correspond to Df1), Df2), Df3) and Df4)

21 Cf. AQUINAS, Exp. Post., Lib. II, lect. 7, p.199, lin. 156-152.22 Cf. ibid., p. 199, lin. 160-162.23 Cf. ibid., p. 199, lin. 164-168.24 Cf. ibid., p. 199, lin. 139-154.


above25 . Of these four accounts only the last three can be properly characterisedas definitions. The first i.e. Df1) is not a definition, strictly speaking, butonly a formula (ratio) of the thing which falls short of definition in the strictsense of the term26 . This also explains why Df1) is not mentioned in Aristotlesfinal list at 94a11-14, which is confined to formulae which can be properlydescribed as definitions. What distinguishes definitions from other kinds ofaccount is the fact that definitions signify the essence of a thing, i.e. presumablythey mention at least some of its essential features27 . Thus, from Aquinassreading there clearly emerges that Df1) however it should be furthercharacterised is an account of a thing which does not mention any of itsessential features. Since Df1) is, on Aquinass reading as well, the mostproblematic case I shall first say a few words about the other three kinds offormulae, i.e. the formulae which can be properly characterised as definitions.Then I shall try to explain how Aquinas understands Df1).

Now, Df4) is taken to be an indemostrable definition of immediateterms28 . As I said, I am not chiefly interested in this kind of definition andso we can leave it once again out of our consideration. Aquinassunderstanding of the relation between Df2) and Df3) is not particularlyproblematic. Df2) is the full, scientific definition of the essence of a thing,i.e. the definition which contains the cause of a things existence29 . Such adefinition can be obtained by re-arranging in an appropriate way the termsof a special kind of syllogism, the syllogism which shows what a thing is.Thus, Df2) is a formula which differs from a syllogism (i.e. the syllogismwhich shows what a thing is) only for the position of the terms involved30 .Df3) is precisely the conclusion of a syllogism showing what a thing is31 .Aquinas characterises Df3) as a definition which only signifies the essenceof a thing32 . What he means is that, like Df2) and Df4), Df3) points to someessential features of a thing and so it signifies its essence but, unlikeDf2) and presumably also Df4), it does not express the whole essence of thething defined and so it only signifies the essence, i.e. confines itself topointing to some essential features but not to all of them. What Df3) lacksis a mention of the cause of a things existence, which should be included,

25 Cf. AQUINAS, Exp. Post., Lib. II, lect. 8, pp. 203-204, lin. 91-191.26 Cf. ibid., p. 203, lin. 99-104.27 Cf. ibid., p. 203, lin. 91-92 ; 115-116.28 Cf. ibid., p. 204, lin. 173-183.29 Cf. ibid., p. 203, lin. 133-134 ; 150-155.30 Cf. ibid., p. 203, lin. 133-143 ; 150- 204, 169 ; p. 204, lin. 183-189.31 Cf. ibid., p. 204, lin. 166-169.32 Cf. ibid., p. 204, lin. 191-192.


by contrast, in a full scientific definition. Therefore, there is no doubt thatDf3) must be identified with the second of Ch. 8s two accounts by which wemay come to know the existence of a thing, i.e. the account which graspssomething of the thing by mentioning some but not all of its essentialfeatures. Admittedly, Aquinas in his commentary on Post. An., II, 10 does notmention existence in connection with Df3). However, as we shall see shortly,Aquinass description of Df1), where existence can at best be known onlyaccidentally, makes it perfectly clear that Df3) gives us a true grasp of theexistence of a thing and so should be identified with Ch. 8s second account.

So far, Aquinass reconstruction of Ch. 10 is perfectly in line with AccountA. But what about Df1) ? Should it be straightaway identified with thenominal definition of a thing ? And should nominal definition be identifiedwith Ch. 8s first kind of account, i.e. the account which mentions only theaccidental features of a thing ? In a way, it is difficult to give an unqualifiedanswer to these two questions. Therefore, I prefer to spell out in some detailAquinass understanding of Df1) and see how it can provide answers to thetwo questions I have just mentioned. What is certain about Df1) is thatAquinas does not regard it as a definition in the strict sense of the term. Fordefinitions in the strict sense of the term must signify the essence of a thing,that is, presumably, must mention at least some of the essential features of athing, and Df1) does not mention any of such features according to Aquinassview. Thus, clearly, Df1) at least includes Ch. 8s first kind of account, i.e. theaccount which mentions only the accidental features of a thing. However, forAquinas, accidental accounts are not the only kind of accounts included inDf1). The Dominican Master says in fact that an account or formula of a thingwhich falls short of a definition can be of two different types : either it is anaccount which explains only the meaning of the name of a thing ; or it is anaccount which bears upon some real features of a thing, even though itmentions only its accidental ones33 . Taken at its face value, Aquinass commentseems to distinguish nominal definitions from accidental accounts. Both areformulae that fall short of definition, in that neither of them make referenceto the essential features of a thing. And so both can be classified as instancesof Df1). However, nominal definition, i.e. the definition explaining themeaning of a things name, seems to be a formula that does not mention anyof the real features of a thing, be they accidental or essential, but onlyconfines itself to explaining what the name associated with the thing means.Accidental accounts, by contrast, mention some of the real features of a thing,i.e. some of its accidental properties. They are explicitly identified by Aquinas

33 Cf. AQUINAS, Exp. Post., Lib. II, lect. 8, p. 203, lin. 99-104.


with the first kind of account Aristotle discusses in Ch. 8, i.e. the formula thatgives us only an accidental knowledge of a things existence34 .

So, should we believe that, according to Aquinas, the nominal definitionof thunder or eclipse does not even mention some of the accidental featuresof thunder or eclipse ? I do not think so. On my view, Aquinass distinctionwhen presenting Df1) is a different one, which does not force us to distinguishin the case of eclipse and thunder between nominal definition and accidentalaccount. Let me try to explain my view. First of all, Aquinass point is thatnominal definitions as such have no existential import. In other words, wecan give a nominal definition, i.e. a formula specifying what the name of athing means, both of things that exist and of things that do not exist. So, atleast some nominal definitions do not even mention accidental features of thething defined : for instance the nominal definitions of things that do not existdo not mention any real features of such things, be they accidental oressential, simply because things that do not exist have no real features therefor us to mention. So, if Aquinas believes, as he does, that nominal definitionsdo not have existential import, he is perfectly right to distinguish nominaldefinitions as such from accidental accounts. For surely, at least somenominal definitions are not accidental accounts. There is also another case tobe taken into account. In addition to things that do not exist, there are alsoitems that can be certainly said to exist in some sense, but do not have thesufficient degree of unity for them to be classified as things in the strict senseof the term. Take for instance the case of the Trojan War, which is narratedby Homer in the poem Ylias. There is certainly a sense in which the TrojanWar can be said to be something that exists or existed : it is presumably aseries of events and facts that we group together on account of certainrelations (of causality, temporal proximity and so on) they bear to oneanother. For Aquinas, however, as for Aristotle, the Trojan War does not havethe sufficient degree of unity for it to be described as one thing. Its unity, inother words, is only accidental : even if the parts of Trojan War are somehowrelated to each other, the war as a whole is not one thing possessing certainessential and accidental properties in the same way as the things possessinga non-accidental degree of unity. Of course, there are many things that can betruly predicated of the Trojan War. But such things are not essential oraccidental properties of the Trojan War in the same sense as being an animalor being pale are, respectively, an essential and an accidental property of aman. Thus, Homers poem, even if we take it to be in some sense an accountof the Trojan War, could not really count as an account which mentionsaccidental or essential properties of it, but at best as an account which

34 Cf. AQUINAS, Exp. Post., Lib. II, lect. 8, p. 203, lin. 104-114.


explains in some detail, to be sure the meaning of the name correspondingto the Trojan War, e.g. Ylias. Therefore, since even the nominal definition ofthings that we can say exist in some sense but do not have a sufficient degreeof unity, does not mention any accidental features of the thing defined, itseems that Aquinas is once again right in distinguishing nominal definitionsas such from accidental accounts.

All this, however, does not rule it out that the nominal definition of thingsthat exist and have also a sufficient degree of unity does in fact mention someof their accidental features. After all, it is difficult to imagine a nominaldefinition of an eclipse which mentions none of the characteristics an eclipsereally possesses, should they even be the most superficial and external to itsessence. Thus, I conclude that, according to Aquinas, the nominal definitionof things which are the objects of scientific investigation, that is things thatexist and have a sufficient degree of unity, are accidental accounts of suchthings, which only mention some of their accidental and superficialcharacteristics. In the case of the objects of scientific investigation, Df1) is anominal definition and an accidental account. On this point too, therefore,Aquinass interpretation is in line with Account A. It should be remembered infact that, according to Barnes too, nominal definitions as such do not haveexistential import. We can give a nominal definition of all sorts of things, forinstance of non-existent things, of things that exist but have a very low degreeof unity as well as of proper objects of scientific investigation. Presumably,however, the nominal definition of an object of scientific investigation willpoint to some real features of the thing, although some merely accidental ones.

3. CONCLUDING REMARKS : AQUINAS AND ARISTOTLES ESSENTIALISM

We have seen that the second book of the Posterior Analytics containsseveral remarks on the relation between essence and existence. I have discussedfor instance in some detail Aristotles claim in Chs. 2 and 8 to the effect thatit is impossible to know the essence of a thing before knowing its existence.Another important passage on the relation between essence and existence isPost. An., II, 7, 92b4-11. The text belongs to Aristotles dialectical discussionof the relation between demonstration and definition. One of the main themesof discussion is the question whether definition can be demonstrated, i.e.whether there is a syllogism whose conclusion is a definition. The question along with some others is discussed dialectically in Chs. 3-7 and thenpositively solved in Chs. 8-10. We have briefly touched upon Aristotlessolution to this difficulty. What can figure as a conclusion of a syllogism isonly a partial definition of a thing, i.e. Df3), a definition which only mentionspart of its essence. The full, scientific definition of a thing can never be the


conclusion of a syllogism, but can nonetheless be obtained by re-arrangingthe terms of the syllogism which has as its conclusion a partial definition ofthe thing. Thus, the scientific definition of a thing is not demonstrated, butrather as Aristotle expresses himself revealed through a demonstration.

In the course of his dialectical discussion (II, 7, 92b4-11), Aristotlepresents a particularly interesting argument against the claim that a definitioncan be demonstrated. The structure of the argument is the following. (i) If oneknows what x is, one must know that x exists ; (ia) for if no such a thing as xexists, one cannot know what it is, but can know at most what the name of xmeans ; (ii) thus, if one demonstrates what x is, one thereby demonstrates thatx exists. (iii) But one demontration demonstrates only one thing and (iv) whatx is and that x exists are two different things. (v) Therefore, one cannotdemonstrate what x is. Aristotles argument is dialectical. So, we should notassume that Aristotle necessarily believes in the truth of its premisses or of itsconclusion. However, some elements in the argument seems to correspond toAristotles genuine doctrine. For one thing, the distinction in (ia) between anaccount which explains the meaning of the name of a thing and an accountwhich reveals what a thing is seems to support Account As and Aquinass intuition that nominal definitions have no existential import : in the caseof things that do not exist there is no definition revealing what they are forthey are nothing but only an account which explains the meaning of theirname. For another, Aristotles general point that what something is and thatsomething exists are two distinct objects of knowledge seems to be put tomuch use in Post. An., II, 8, as we have seen. And it is precisely this secondaspect of Aristotles argument which Aquinas picks up on in his commentary :

Just as definition is introduced in order to make known something which is one in that from the parts of a definition there results something which is one perse and not accidentally so it is necessary that demonstration, which usesdefinition as middle term, demonstrate something which is one (...) And thus itis evident that things that are different cannot be demonstrated through one andthe same demonstration. But that a man exists and what a man is are differentthings (Only in the first principle of being, in fact, which is being essentially,being itself and quiddity are one and the same thing ; in all other things, bycontrast, which are beings only by participation, it is necessay that being andquiddity be different). Therefore, it is not possible for someone to demonstratewhat something is and that it exists through one and the same demonstration 35 .

35 Cf. AQUINAS, Exp. Post., Lib. II, lect. 6, p. 194, lin. 35-50 : Sicut diffinitio inducitur admanifestandum aliquid unum, in quantum scilicet ex partibus definition fit unum per se, nonper accidens, ita etiam oportet quod demonstratio, que utitur diffinitione tamquam medio,


Although Aquinas provides a faithfull reconstruction of Aristotlesargument, he also injects into it an ontological consideration which does notseem to be explicitly present in the text. According to the Dominican Master,Aristotles argument suggests that essence and existence are not only twodifferent objects of knowledge but also two distinct ontological principles orconstituents of things. In the first principle, i.e. God, such two costituents areidentical, whereas in all created things essence and existence are two mind-independently distinct constituents. In other words, all creatures display areal, i.e. mind-independent composition of essence and existence. We do notneed to think that Aquinas regards Aristotles passage as an argument for thereal distinction between essence and existence. However, he is clearly of theopinion that the general tenor of the passage as well as Aristotles generaldoctrine in the second book of the Posterior Analytics is quite compatiblewith taking essence and existence to be two ontological components of athing. Thus, even if Aristotles remarks in II, 7, 92b4-11 are not strictlyspeaking an argument for the distinction, they should clearly be taken,according to Aquinas, at least as an intuitive ground for such an argument.

But is Aquinas right ? Does the spirit of Aristotles doctrine in Post. An., II,go in the direction of an ontological distinction between essence and existence ?Of course, a detailed analysis of Aquinass celebrated doctrine goes muchbeyond the scope of this paper. I shall confine myself to indicating the generalreasons why I believe that Aquinass interpretation is not right and Aristotlesgeneral doctrine in Post. An., II, goes rather in the opposite direction. Now,Aquinass view on essence and existence clearly represents a major departurefrom a certain essentialist treatment of existence. According to essentialim,to exist for a thing simply means to belong to a certain kind, i.e. to be such-and-such kind of thing. A things membership in a kind and its existing are nottwo distinct facts about a thing : when a thing exists, its existence can betotally explained by its being a member of a certain kind. And being a memberof a certain kind means possessing a certain essence, i.e. displaying certainessential characteristics and behaving in the way appropriate to the kind thething belongs to. It should be noted that the essentialist view is a view aboutontology and not about epistemology, i.e. it concerns the way things are andnot the way we come to know things. In other words, it may well be the case

unum aliquid demonstret (...) et ita patet quod per unam et eandem demonstrationem nonpossunt diversa demonstrari ; set aliud est quod quid est homo et esse hominem (in solo enimprimo essendi principio, quod est essencialiter ens, ipsum esse et quiditas eius est unum etidem ; in omnibus autem aliis, que sunt entia per participationem, oportet quod sit aliud ens etquiditas eius) ; non est ergo possibile quod eadem demonstratione demonstret aliquis quid estet quid est . The translation is mine.


that to know that something is and to know what something is are two distinctpieces of knowledge we come to acquire separately and at different stages.However, regardless of how we come to know the essence and the existence ofa thing, the fact remains that a things existence simply consists in its belongingto a certain kind, and so existence as such does not add anything to theontological structure of a thing. Aquinass view, by contrast, presupposes amore metaphysically loaded notion of existence. For him, to exist for a thingdoes not simply mean to possess a certain essence and so to be a member of acertain kind. Of course, everything that exists does have a certain essence andis a member of a certain kind, but the existence of a thing is a further fact thatcannot be totally explained by the things having an essence or being memberof a certain kind, but rather requires further explanation. In other words, inaddition to a things having an essence we also need to explain why a thingpossessing the properties which characterise a certain essence actually exists.Thus, on Aquinass view, every existing thing (with the exception of God) doesnot only possess an essence, but also a further principle explaining why suchan essence is an actually existing thing. And this further principle is preciselythe act of being or existence. In contrast with the essentialist view, therefore,Aquinas maintains that there is a certain parallelism between the ontologicalstructure of things and the way we come to know them. Just as we come to knowwhat something is and that a thing exists separately and at different stages, soessence and existence are two distinct ontological constituents of a thing, inthat they explain two distinct and irreducible facts about a thing, i.e. the factthat a thing possesses certain essential characteristics and the further fact thata thing with certain essential characteristics actually exists.

Does Aquinass ontological reading of the essence/existence distinctionfind any support in Aristotles doctrine in the second book of the PosteriorAnalytics ? I do not think so. In general Aristotle has very little to say aboutexistence as such. He seems to hold on to a strictly essentialist treatment ofexistence, according to which to exist for a thing simply consists in beingsuch-and-such a kind of thing and so in possessing a certain essence. Hisfamous doctrine, for instance, that there are different senses or ways ofexisting should be probably read according to an essentialist paradigm. Thereare different ways of existing precisely because there are different kinds ofthing, the kinds corresponding to the categories. To exist for a substance is tohave the essential characteristic traits of a substance, and to exist for amember of any other category means to have the essential, characteristictraits of the category in question, be it a quantity, a quality or any other kindof thing. The sense in which a substance exists is primary and that in whichthe members of the accidental categories exist is secondary becauseaccidents depend on substance in a way in which substances do not depend


on accidents. Admittedly, such a dependence also involves what we may callexistential aspects such as for instance the fact that accidents exist insubstances whilst substances do not exist in accidents. However, the reasonto introduce a distinction among different senses of being or existing in thefirst place is motivated by essentialist considerations : there are differentsenses of existing precisely because there are different kinds of thing.

Now, the second book of the Posterior Analytics is the place in Aristotlescorpus where he distinguishes more sharply between essence and existence.However, Aristotles considerations do not go farther than the epistemologicallevel. He says that to know that something exists and to know what somethingis are in fact two distinct pieces of knowledge : we must know that somethingexists before we come to know the complete essence of a thing. But nothingof what Aristotle says seems to have implications for the ontological level. Inother words, nothing of what he says seems to suggest that to exist for a thingconsists in anything more than being a member of a certain kind and that thefact that something exists needs to be explained by having recourse toprinciples other than the essence itself36 . On the contrary, even some of thethings Aristotle says about our knowledge of the existence of a thing seem tobetray an essentialist treatment of existence. In Post. An., II, 8 and 10 heremarks that those who possess only an accidental account of a thing, i.e. anaccount mentioning only some of a things accidental features, cannot really

36 Of course, in one case, i.e. the case of substances, the fact that a thing comes into existencemust be explained by having recourse to some further principles in addition to the thingsessence, for instance the agent that brings about the generation of the thing in question. ForAristotles view is clearly that the efficient cause of substances does not enter into the accountof their essence. However, it must be observed that the things Aristotle investigates into in thePosterior Analytics are not only substances (and perhaps not chiefly substances), but also eventssuch as thunder or eclipse. And Aristotle himself suggests in Post. An., II, 10 that the agent thatbrings about a thunder or an eclipse is at least part of these things essence (a doctrine Aristotleseems to re-state in Met., Z, 17 as well). Moreover, even if we confine ourselves to the case ofsubstances, to say that a substance comes into existence in virtue of principles other than itsessence is different from saying that when a thing exists it is actually composed of two differentprinciples, one accounting for the things possessing certain essential features and the otheraccounting for such a things actually existing. In other words, the distinction between essenceand existence mainly concerns the static, ontological structure of things and not the way inwhich they come into being. Finally, it is also true that a thing comes into existence in virtue ofprinciples that bear a certain relation to its essence. For the agent that brings a thing into beingtransmits to the product the form which is the essence of the product itself. If one takes it asI was doing that for Aristotle the essence of sensible substances only includes their form, theonly distinction in the structure of things which Aristotle makes room for is that between theessence of a thing and its matter. However, Aristotle never connects such a distinction to thedistinction between essence and existence and, given his general essentialist treatment ofexistence, there are no reasons to think that he should.


be said to know the existence of the thing in question. Actually, only thosewho come to know the existence of a thing by grasping something of the thingitself, i.e. by an account which mentions some of a things essential features,can be said to properly know the existence of a thing. Aristotles distinctionclearly suggests that our knowledge of the existence of a thing is somehowconnected with our identifying the thing as a member of a certain kind. Onlyby grasping some of the things essential features and so only by recognisingthe thing as an instance of a certain kind, can we come to have a properknowledge of its existence. Of course, our knowledge of a things existencedoes not involve for Aristotle, but rather excludes a full understanding of athings essence. So, also our identifying the thing as an instance of a certainkind will be only partial at the moment at which we actually know itsexistence. For instance, we may come to know that a thing exists only byknowing the genus to which such a thing belongs. However, to know theexistence of a thing by grasping its genus or some other of its essentialfeatures still counts as an instance of knowing the thing as a thing of a certainkind. Further investigation will specify and determine the nature of the kinda thing belongs to, but it is clear that from the very moment we properly knowthat a thing exists we also know that such a thing belongs to a certain yetto be fully specified kind. Thus, Aristotles doctrine seems to be that to existfor a thing simply means to be such-and-such, i.e. to be a thing of a certainkind, even if we come to determine the precise nature of the kind the thingbelongs to only by degrees.

Aristotles essentialist line is confirmed by another piece of doctrine wehave touched upon in Section I, namely the view that the essence of a thingis the cause or explanation of its existence. In Post. An., II, 8 and 10, Aristotletells us that there are accounts of a thing that mention some of its essentialfeatures but not its cause or explanation. This implies that, technically, thecause or explanation of a things existence is only part of a things essence anddoes not straightaway identify with it. However, Aristotles considerations inPost. An., II, 2, suggest that there must be a less technical sense in whicha things essence as a whole is the cause or explanation of its existence. For inCh. 2 Aristotle simply identifies the essence of a thing with the cause of itsexistence. Presumably, Aristotles remarks should be read in the light of hisessentialist treatment of existence. To exist for a thing simply consists inbeing such-and-such a thing, i.e. in being a member of a certain kind. Thus,once we know the essence of a thing, there is no other cause or explanationof its existence for us to look for. For the cause or explanation of that thingsexistence simply is that things having the essence it has and so being amember of the kind it belongs to. No additional factor is required to accountfor a things existing let alone some extra ontological constituent.


I conclude, therefore, that Aquinass ontological reading of Aristotlesdistinction between knowing the essence and knowing the existence of a thingis not warranted by the text of the Posterior Analytics. In other words, Aquinasreads back into Aristotles text a doctrine which is one of the cornerstones ofhis metaphysical thought. This aspect, however, does not diminish the meritsof his otherwise careful and insightful reading of Aristotles doctrine in thesecond book of the Posterior Analytics.

ABSTRACT

The paper illustrates Aquinass reading of Aristotles doctrine of definition in Post.An., II, 8-10, with particular reference to the distinction between nominal and realdefinition. The analysis of Aquinass interpretation is set against the background ofthe contemporary debate over Aristotles understanding of the relations betweendemonstration and definition. The main thesis the paper argues for is that Aquinasprovides a consistent interpretation of Post. An., II, 8-10, which mainly centres on theclaim that nominal definitions do not have existential import. In the final part of thearticle, some light is also shed on how Aquinas exploits Aristotles considerations inthe second book of the Posterior Analytics to give further support to his distinctionbetween essence and existence.

Documents

Il tema della verità nell’Expositio Libri Peryermenias di Tommaso d’Aquino - Galluzzo