AbstractJaakko Hintikka (1998) has argued thatclarifying the notion of abduction is thefundamental problem of contemporaryepistemology. One traditional interpreta-tion of Peirce on abduction sees it as arecipe for generating new theoretical discov-eries. A second standard view sees abductionas a mode of reasoning that justifies beliefsabout the probable truth of theories. Whileeach reading has some grounding in Peirce’swritings, each leaves out features that arecrucial to Peirce’s distinctive understandingof abduction. I develop and defend a thirdinterpretation, according to which Peircetakes abductive reasoning to lead to judg-ments about the relative pursuitworthinessof theories; conclusions that can be thor-oughly disconnected from assessments oftruth-value. Even if Peirce’s use of “abduc-tion” slides around among each of thesethree importantly different though poten-tially compatible senses, this neglected thirdunderstanding makes sense of a large num-ber of Peirce’s remarks and directs our atten-tion to the cognitive structure of judgmentsthat scientists face after the initial proposalof explanatory hypotheses but prior to theirexperimental testing; a topic which shouldbe of interest to contemporary philosophersof science.

Keywords: abduction, discovery, economy,inference to the best explanation,justification, Peirce, pursuit,pursuitworthiness, retroduction, scientificmethod

1 IntroductionPeirce famously distinguished betweenthree kinds of inference: deduction, induc-tion, and abduction. However, understand-

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ing exactly what Peirce had in mind by abduction or the closely relatedterms “retroduction,” “hypothesis,” and “presumption” has proved tobe controversial. There remains a sense among many Peirce scholarsthat “Peirce’s concept of abduction is still poorly understood” (Chias-son 2001). Some have even suggested that “[t]he proper conclusionmight be that he lacked a clear and consistent picture of what is to beinferred in an abduction, or, for that matter, a clear distinction betweenbelief and other practical attitudes” (Kapitan 1992, 15; see also Frank-furt 1958b, 593). This situation is regrettable, because Peirce thoughtof abduction as the heart of his philosophy and as one of his chief con-tributions to scientific methodology:

If you carefully consider the question of pragmatism you will see thatit is nothing else than the question of the logic of abduction. That is,pragmatism proposes a certain maxim which, if sound, must renderneedless any further rule as to the admissibility of hypotheses to rankhypotheses, that is to say, as explanations of phenomena held as hope-ful suggestions; and, furthermore, this is all that the maxim of prag-matism really pretends to do. (CP 5.196)

In this essay, I identify two main traditions of interpretation ofPeirce on abduction, neither of which, I will argue, gets things quiteright. One traditional interpretation of Peirce on abduction sees it as arecipe for generating new theoretical discoveries. A second standard viewsees abduction as a mode of reasoning that justifies beliefs about theprobable truth of theories. While each reading has some grounding inPeirce’s writings, each leaves out features that are crucial to Peirce’s dis-tinctive understanding of abduction. I develop and defend a thirdinterpretation, according to which Peirce takes abductive reasoning tolead to judgments about the relative pursuitworthiness of theories; con-clusions that can be thoroughly disconnected from assessments oftruth-value. This revisionist reading of Peirce, which sees practical or“economic” considerations as central, should lead philosophers of sci-ence to rethink the relationship between Peirce’s abduction and con-temporary discussions of inference to the best explanation. Even ifPeirce’s use of “abduction” slides around among each of these threeimportantly different though potentially compatible senses, this neg-lected third understanding makes sense of a large number of Peirce’sremarks and directs our attention to the cognitive structure of judg-ments that scientists face after the initial proposal of explanatoryhypotheses but prior to their experimental testing; a topic which shouldbe of interest to contemporary philosophers of science.

When seen as an attempt to describe the way that practical consid-erations enter into experimental methodology, we can draw on Peirce’ssustained reflection on abduction—which spanned more than fifty

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years—to deepen our understanding of several issues of interest to con-temporary philosophy of science. In particular, his discussion con-tributes to the characterization of the kinds of cognitive judgments thatscientists make in response to the question—so pressing at the earlystages of inquiry—“which hypotheses should we develop further?” Alook at examples from actual science shows pursuitworthiness judg-ments playing an important role in scientific practice in ways that fitfairly well with the reasoning processes Peirce describes. If Peirce isright, judgments about whether a hypothesis is worth pursuing turnout to be far more important to scientific practice than philosophershave typically recognized. Far from compromising the objectivity ofscientific inquiry, on Peirce’s view, non-epistemic factors play a strategicrole in Peirce’s plan for the efficient advancement of science.1

2 Discovery and Justification: Too Early, Too LateTwenty four years after Peirce’s death, Hans Reichenbach distinguishedbetween contexts of justification and contexts of discovery in Experi-ence and Prediction (1938), a distinction that has colored a great deal ofsubsequent Peirce interpretation. In the years to follow Reichenbach’sbook, philosophers of science wielded this influential distinction as away of carving up disciplinary boundaries. The rational reconstructionof historical episodes and analysis of concepts relevant to evidentialsupport fell properly within the domain of philosophy of science,where perhaps philosophers could identify a set of formal rules govern-ing rational processes of theory appraisal. Those issues relegated to thecontext of discovery could be dismissed as beyond the purview of epis-temology or methodology, with the thought that there can be no recipefor innovation (for some typical expressions of this attitude, seeReichenbach 1951, 251; Braithwaite 1953, 31fn; Popper 1959, 20fn;Hempel 1966). Episodes of creative insight, such as the dream about asnake biting its tail that inspired Kekulé to propose a hypothesis aboutbenzene’s structure as a hexagonal ring or stories about research skewedby personal bias, could be left for a few descriptively-minded historiansand social psychologists to explore.2

2.1 The Generative Interpretation of AbductionPost-war thinkers like Norwood Russell Hanson took up the gauntletagainst this tendency to dismiss the idea that there could be anythinglike a “logic of discovery,” hoping to show that the reasoning processesleading to discoveries frequently display an interesting rational struc-ture. Perhaps it is not surprising, then, that Hanson would read Peirceas offering the kind of “pattern of discovery” that he sought.

When Hanson (1958, 85–86) interprets Peirce on abduction, hecites remarks from Peirce’s Harvard series of “Lectures on Pragmatism”(March 26 to May 17, 1903) that pertain to the origin of hypotheses,

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such as “[a]bduction consists in studying facts and devising a theory toexplain them” (CP 5.145) and “[a]bduction is the process of formingan explanatory hypothesis” (CP 5.171).

Call this first way of interpreting Peirce’s theory of abduction, whichsees abduction as a reasoning process resulting in the discovery of newhypotheses, the Generative Interpretation. According the GenerativeInterpretation, abduction is a form of guessing by which we come upwith explanatory hypotheses to be tested. One can find places wherePeirce seems to conceive of abduction as the act of proposing a hypoth-esis to explain an otherwise surprising phenomenon; places wherePeirce says things like: “This synthesis suggesting a new conception orhypothesis, is the Abduction” (The Essential Peirce, vol. 2, 287).3

Hanson draws on Peirce to argue that Kepler’s reasoning from TychoBrahe’s observational data to the hypothesis that planetary motion iselliptical was not just flash of insight or inexplicable genius. As Hanson(1958, 85) has it, hypothesis creation is a rational process (if not quitea step-by-step process, at least one that is more constrained and con-trolled than Popper made it out to be). Hanson writes: “Kepler typifiesall reasoning in physical science. Would it have required so much time,and genius to ‘observe’ the elliptical orbit in Tycho’s data?” Hansonthinks not. Once Kepler had discerned that initial pattern in the data,Kepler’s explanatory hypothesis follows as a straightforward piece ofabductive reasoning.

It required no genius to take Kepler’s idea and try it for other planets.Kepler never modified a projected explanation capriciously; he alwayshad a sound reason for every modification he made. . . . This is thegreatest piece of Retroductive reasoning ever performed. (Hanson1958, 85)

Similarly, Fann’s extended treatment of Peirce’s theory of abduction hasit that “Natural scientists do not ‘start from’ hypotheses. They startfrom data. Peirce’s theory of abduction is concerned with the reasoningwhich starts from data and moves toward hypotheses” (Fann 1970, 5).Other examples of the Generative Interpretation include Nickles(1985) and Joseph Brent’s (1998) biography of Peirce, which empha-sizes Peirce’s idea that abduction is a form of guessing by which wecome up with explanatory hypotheses to be tested. This approach isalso found, to varying degrees, in Davis (1972), Shanahan (1986),Roth (1988), Turrisi (1990), and—despite more attention to economicconsiderations—Brown (1983).

Peirce does indeed have interesting views about hypothesis genera-tion that can serve as fertile ground for reflection on the process of dis-covery. For example, Peirce thought that the kind of solid progress wehave made in science, at the rate we have made it, suggested that wehave a kind of instinctive knack for generating good explanations:

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Consider the multitude of theories that might have been sug-gested. . . . Think of what trillions of trillions of hypotheses might bemade of which only one is true; and yet after two or three or at thevery most a dozen guesses, the physicist hits pretty nearly on the cor-rect hypothesis. (CP 5.172; see also 7.220)

Peirce thought that, as a product of evolution, “man has a certainInsight, not strong enough to be oftener right than wrong, but strongenough not to be overwhelmingly more often wrong than right” (CP5.173). Moreover, Peirce does talk about this insight in the context ofdiscussing abduction.

But, as an understanding of abduction, there are several reasons tothink that the Generative Interpretation mischaracterizes crucial fea-tures of Peirce’s view. First, Peirce sees this “insight” is an instinctivecapacity, not classified with the “powers of reason.” Rather, it is a kindof nose for the truth that belongs “to the same general class of opera-tions to which Perceptive Judgments belong” (CP 5.173). WhereasPeirce insisted again and again over the course of his career that abduc-tion was a distinctive mode of inference, these spontaneous and instinc-tive judgments are apparently non-inferential, being such that “[e]vennow [the scientific innovator] cannot give any exact reason for his bestguesses” (CP 5.173).4

Second, and more importantly, in Peirce’s canonical statement of theform of abductive inferences (CP 5.189) explanatory hypotheses figurein the premises, thus presupposing that we already have available a set ofalternatives about which to make some further judgment. Peirceoffered the following schema for abduction in the Harvard lectures in1903:

The surprising fact, C, is observed;But if A were true, C would be a matter of course,Hence, there is reason to suspect that A is true. (CP 5.189)5

As Frankfurt observed in 1958, this doesn’t look like the sort of reason-ing that Kepler can use to generate a novel hypothesis. Rather, assumingsome potential explanation is available, prima facie, it looks like thisreasoning is invoked as a means of providing fallible evidential supportto explanatory hypotheses that go beyond the observational data. Hereis an inference with a “perfectly definite logical form” (CP 5.188);albeit a form that looks worrisomely like the deductive fallacy of affirm-ing the consequent. This leads us to the Justificatory Interpretation.

2.2 The Justificatory Interpretation of AbductionThe standard view in the literature on inference to the best explanationsees Peirce’s abduction as a non-deductive mode of inference that justi-

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fies claims concerning the probable truth of explanatory hypotheses.Call this the Justificatory Interpretation of Peirce’s theory of abduction.As Misak puts it, “Peirce was the first to put a name to, and discuss . . .inference to the best explanation, or what Peirce called ‘abduction’”(Misak 2000, 336). Peter Lipton’s book Inference to the Best Explanation(2004, 56) now in its second edition, makes the standard obligatoryreference to Peirce’s 1903 schema (CP 5.189). Ernan McMullin (1992)appropriates the term “retroduction” for a mode of reasoning fromeffects to cause, which, while not rule-governed, provides the epistemicjustification for a great many scientific claims.6 The hallmark of the Jus-tificatory Interpretation is that abduction is an ampliative mode ofinference that gives us some reason to think that the conclusion is trueor approximately true or probably true or at least more likely to be truethan any available alternatives.

I am not at all sure these authors are talking about the same thing asPeirce. Indeed, the identification of abduction with inference to thebest explanation can only be made by ignoring features that are crucialto Peirce’s understanding.7 When we look beyond Peirce’s schema, tohis other remarks about abduction, he says things that are much harderto square with the Justificatory Interpretation. Particularly challenginghere are numerous remarks (to be discussed below) which suggest thatthe conclusions of abductive reasoning can be thoroughly disconnectedfrom assessments of truth-value. For Peirce, “abduction commits us tonothing. It merely causes a hypothesis to be set down upon our docketof cases to be tried” (CP 5.602).

As I see it, the Justificatory Interpretation confuses Peirce’s notion ofabductive reasoning with questions about the justification of theoriesthat, for Peirce, would fall under inductive reasoning, broadly con-strued. For Peirce:

Abduction . . . is merely preparatory. It is the first step of scientificreasoning, as induction is the concluding step. Nothing has so muchcontributed to the present chaotic or erroneous ideas of the logic ofscience as failure to distinguish the essentially different characters ofdifferent elements of scientific reasoning (CP 7.218).

Peirce reserves the term “Induction” for the reasoning involved in the-ory confirmation, where the picture of evidential support he endorses isa fairly traditional hypothetico-deductive model of confirmation or fal-sification (see Rescher 1978, 3). This process of testing theoretical pre-dictions and subsequently evaluating them epistemically comes afterjudgments have been made about which hypotheses are worth pursuingand, indeed, only after the experiments have been performed.

That which is to be done with the hypothesis is to trace out its conse-quences by deduction, to compare them with the results of experiment

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by induction, and to discard the hypothesis, and try another, as soon asthe first has been refuted; as it presumably will be. How long it is beforewe light upon the hypothesis which shall resist all tests we cannot tell;but we hope we shall do so at last. (CP 7.220; emphasis added)

Contrast this with what Peirce says about abduction:

[The inferential step that Peirce calls “abduction”] will include a pref-erence for any one hypothesis over others which would equallyexplain the facts, so long as this preference is not based upon any evidencebearing on the truth of the hypotheses, nor on any testing of the hypothe-ses, after having admitted them on probation. I call all such inference bythe peculiar name, abduction, because its legitimacy depends on alto-gether different principles from those of other kinds of inference. (CP6.524–6.525; emphasis added).

3 Pursuitworthiness

3.1 The Pursuitworthiness Interpretation of AbductionI want to explore a third way of understanding Peirce on abduction,which I shall call the Pursuitworthiness Interpretation. Although Peircedoes not use the term “pursuit” in this connection, which—so far as I cantell—we owe to Larry Laudan (1977), the designation is apt for severalreasons.8 First, it directs our attention to a stage of scientific inquiry thatlies between the initial discovery or proposal of hypotheses and their laterexperimental justification. Second, it shifts the focus away from epistemicappraisal, placing the emphasis instead on judgments about whether atheory is worth investigating and developing further.

According to the Pursuitworthiness Interpretation, abductive rea-soning is the label Peirce gives to his systematic attempts to think aboutthe qualities that factor into decisions about whether investigating anidea looks promising or seems worthwhile. Abductive reasoning makespractically grounded comparative recommendations about which availablehypotheses are to be tested:

[T]here is only a relative preference between different abductions;and the ground of such preference must be economical. That is to say,the better abduction is the one which is likely to lead to the truthwith the lesser expenditure of time, vitality, etc. (NEM4, 37–38)

Peirce is offering a view about the way that “economic” or practicalconsiderations enter into our answer to the question—so pressing atthe early stages of inquiry—“which hypotheses should we develop fur-ther?” While questions about whether a hypothesis is likely to be trueor more probable than its competitors given the evidence are not irrel-evant to such decisions, such estimates are often not the most impor-tant factor in these judgments.

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On this interpretation, Peirce’s discussion of the economics ofresearch plays a crucial role; another aspect of his thought that, asBurch 2006 points out, “has been insufficiently explored by Peircescholars” (Burch 2006). I associate my own reading most closely withthe views of a handful of scholars, such as Frankfurt (1958), Curd(1980), Thagard (1981), Achinstein (1993), and Kapitan (1992,2000), who have taken seriously Peirce’s insistence that “the rules of sci-entific abduction ought to be based exclusively upon the economy ofresearch” (Peirce CP 7.220, fn. 18). If we want a full picture of Peirce’sunderstanding of abduction, we must direct our attention to whatPeirce insists “really is in all cases the leading consideration in Abduc-tion, which is the question of Economy—Economy of money, time,thought, and energy” (CP 5.600). Abduction is, for Peirce, a researchstrategy. For researchers, the “main problem is, how, with a givenexpenditure of money, time, and energy, to obtain the most valuableaddition to our knowledge” (CP 7.140).

3.2 Practical Recommendations and the Role of Economic Considerationsin Abductive ReasoningWe can begin to get a handle on the sort of reasoning Peirce envisionedby asking about what sort of conclusions abductive inferences are sup-posed to issue in.

There is a sharp contrast, for example, with William Whewell, whothought that a “consilience of inductions” should “impress us with theconviction that the truth of our hypothesis is certain” (Whewell 1847,Vol. II, 65). Peirce, of course, did not claim anything so strong forabductive reasoning. Even in the schema that the Justificatory Interpre-tation takes as canonical, we saw that Peirce thinks the proper attitudeto take toward the potential explanation of a surprising fact is that wehave some reason to suspect that the best theory is true. Elsewhere Peircehas it that:

The conclusion of an abduction is problematic or conjectural, but itis not necessarily at the weakest grade of surmise, and what we callassertoric judgments are, accurately, problematic judgments of a highgrade of hopefulness. (CP 5.192)

Peirce, of course, is a thoroughgoing fallibilist. But the point here is notjust that we must trade dreams of certainty for Locke’s “twilight ofprobability” (Locke 1689, 652). Peirce has something deeper in mindwhen he says of abduction that “[t]o assert the truth of its conclusionever so dubiously would be too much” (MS 692: 26).

The more radical and important point here is that for Peirce the con-clusions of abductive reasoning can be entirely non-epistemic in character;they are not, in the first instance, evaluations of the likely truth of ahypothesis. We tend to read remarks like “Retroduction does not afford

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security. The hypothesis must be tested” (CP 6.470) as epistemichedges or qualified truth-claims. Yet, Peirce saw his own thinking aboutabduction as having undergone at least one substantial shift in 1898,after which he took it that “the reasoning with which I was there deal-ing [Peirce’s 1883 “A Theory of Probable Inference”] could not be thereasoning by which we are led to adopt a hypothesis” and that “proba-bility proper had nothing to do with the validity of Abduction” (CP2.102; see also CP 8.227).9 In the Cambridge Lectures on “Reasoningand the Logic of Things” (1898), Peirce has it that with retroduction(abduction):

Not only is there no definite probability to the conclusion, but nodefinite probability attaches even to the mode of inference. We canonly say that the Economy of Research prescribes that we should at agiven stage of our inquiry try a given hypothesis, and we are to hold itprovisionally as long as the fact will permit. There is no probabilityabout it. It is a mere suggestion which we tentatively adopt. (PeirceNEM4, 184; emphasis added)

In what I take to be one of the most lucid and well-supported dis-cussions of Peirce on abduction available, Tomis Kapitan (1992) force-fully argues that the conclusions of abductive inferences are practicaldirectives (or even interrogatives).10 Rather than statements about whatwe are entitled or required to believe, abductions yield recommendationsabout what courses of action to pursue given our values and given theinformation and resources at our disposal. That is the crucial interpre-tive move and, once one has made it, one can find abundant textualsupport for the Pursuitworthiness reading in Peirce’s writings.

Indeed, in 1903 Peirce explicitly says that for abductions, “[t]heconclusion is drawn in the interrogative mood” (The Essential Peirce,vol. 2, 287). Although Peirce sometimes speaks of the conclusions ofabductions as “plausible,” we have already seen that readers should notbe misled into thinking these are judgments of probability.

By plausible I mean that a theory that has not yet been subjected toany test, although more or less surprising phenomena have occurredwhich it would explain if it were true, is in itself of such a character asto recommend it for further examination (CP 2.662; emphasis added).

We find here both premises that figure into Peirce’s schema (CP 5.189),the surprising phenomena and the explanatory hypothesis. Yet, theconclusion is a recommendation to pursue the hypothesis; to develop it,to work it out its consequences in detail, and to check whether it fitsour precise experimental observations.

The content of the conclusion is not the hypothesis itself, but rathera suggestion that we look into it. For Peirce:

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The best hypothesis, in the sense of the one most recommending itselfto the inquirer, is the one which can be the most readily refuted if itis false. This far outweighs the trifling merit of being likely. (CP1.120; emphasis added)

The reason that “abduction is, after all, nothing but guessing” (CP 7.219)is not because it is a method for generating hypotheses any more than itis a method for justifying them. Rather, Peirce is considering “what prin-ciples should guide us in abduction, or the process of choosing a hypoth-esis . . . . which we must embrace at the outset, however destitute ofevidentiary support it may be” (CP 7.219; my emphasis). As he sees it:

If there be any hypothesis which we happen to be well provided withmeans for testing, or which, for any reason, promises not to detain uslong, unless it be true, that hypothesis ought to be taken up early forexamination. Sometimes the very fact that a hypothesis is improbablerecommends it for provisional acceptance on probation. (CP 6.533;emphasis added).

Notice just how different this is from the usual kind of conclusioninferred by contemporary proponents of inference to the best explana-tion, where the interest is in epistemic justification or evidential support;in conclusions to the effect that the hypothesis in question is true, orapproximately true, or probable, or more probable than any of the avail-able competitors, and the like. Peirce is confident that the scientific com-munity will get there in the long run, to be sure. But would-be epistemicevaluators must be patient. At best, it is only after testing that we canbegin to make epistemic evaluations of any real importance. His concep-tion of science as a cooperative social endeavor lends itself to a view of sci-entific progress in terms much longer than the lifespan of any individual.

With science . . . a problem started today may not reach any solutionfor generations. The man who begins the inquiry does not expect tolearn, in this life, what conclusion it is to which his labors are tending.Strictly speaking, the inquiry will never be completely closed. Evenwithout any logical method at all, the gradual accumulation of knowl-edge might ultimately bring a sufficient solution. Consequently, theobject of logical method is to bring about more speedily and at lessexpense the result which is destined, in any case, ultimately to bereached, but which, even with the best logic, will probably not come inour day. Really the word belief is out of place in the vocabulary of science.(CP 7.186; my emphasis; see also CP 7.220fn18 where a similar pointis made about the role of abduction in scientific investigation)

The recurring temptation for each generation is to see itself as occupy-ing a late stage of inquiry—a stage in which our theories have pro-gressed to a point sufficiently close enough to what we might accept in

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the long run—a stage late enough that the most important sort of ques-tion to ask about it is whether or not it is true or whether it (or some-thing like it) is (likely) to be (approximately) true.

The judgments scientists face in the early stages of inquiry, at least,are of a different character. Reflecting on abduction in 1902, Peircewrites:

Any hypothesis which explains the facts is justified critically. Butamong justifiable hypotheses we have to select that one which is suit-able for being tested by experiment. There is no such need of a subse-quent choice after drawing deductive and inductive conclusions.(NEM4, 62; emphasis added)

Peircean abduction focuses on selecting which of the availablehypotheses investigators should pursue; on decisions that researchersface before they are in a position to make any sort of reliable assessmentabout the truth-values of the alternatives. With the right method, wecan hope to carry out our investigations more efficiently:

The aid which a correct logic can afford to science consists inenabling that to be done at small expenditure of every kind which, atany rate, is bound to get done somehow. The whole service of logic toscience, whatever the nature of its services to individuals may be, isthe nature of an ecomony. . . . therefore, in scientific investigationabduction can subserve no other purpose than economy. (CP7.220n.18)

Understanding the type of conclusion we are after helps to clarifythe decision problem that Peirce takes experimentalists to face at theearly stages of inquiry in such a way that puts more distance betweenPeirce and the traditional interpretations of abduction. For Peirce theproblem is not primarily, as the Generative Interpretation would haveit, coming up with explanations. He writes:

Proposals for hypotheses inundate us in an overwhelming flood,while the process of verification to which each one must be subjectedbefore it can count as at all an item, even of likely knowledge, is sovery costly in time, energy, and money—and consequently in ideaswhich might have been had for that time, energy, and money, thatEconomy would override every other consideration even if there were anyother serious considerations. In fact there are no others. (CP 5.602;emphasis added)11

Here, Peirce takes it for granted that there are a number of availablealternative explanatory hypotheses that we might test. What Peirce isafter is “a maxim which looks only to possibly practical considerations”that will tell us “how we ought in consistency to shape our practical

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conduct” (CP 5.196). A resolution to act in a particular way arguablyeven lacks truth-value. If the conclusion has the status of a recommen-dation, then, although the conclusion won’t be the sort of thing that istrue or false, the course of action recommended can be vindicated ornot. Alternatively, one might argue that it is an objective matterwhether pursuing a given hypotheses will turn out to be an efficientway of achieving the goal of providing a correct theoretical account ofthe phenomena, but whether the hypothesis is judged in hindsight tobe worth pursuing or not is distinct from whether there are now reasonsto pursue the hypothesis.

If abduction issues in recommendations about which of the avail-able theories to pursue, how do economic considerations factor in?Peirce writes:

He therefore accepts that theory so far as to give it a high place in thelist of theories of those phenomena which call for further examina-tion. If this is all his conclusion amounts to, it may be asked: Whatneed of reasoning was there? Is he not free to examine what theorieshe likes? The answer is that it is a question of economy. If he exam-ines all the foolish theories he might imagine, he never will (short ofa miracle) light upon the true one. (CP 2.776)

The task that researchers face is to come up with a plausible ranking ofthe priority with which available potential explanations are to be tested.When we examine the structure of the reasoning involved in judgmentsmade in the context of pursuit or practical inquiry, we see that factorslike our time, resources, and value of the estimated payoff in compari-son to other courses of action all play a major role in such reasoning.Peirce writes:

Having a certain fund of energy, time, money, etc., all of which aremerchantable articles to spend upon research, the question is howmuch is to be allowed to each investigation; and for us the value ofthat investigation is the amount of money it will pay us to spendupon it. (CP 1.122)

If we estimate that testing the hypothesis will be easy, of potential inter-est, and informative, then we should give it a high priority.12 These rec-ommendations are offered in the mode of wise advice (CP 6.535); as aform of “intelligent guessing [that] may be expected to lead us to theone which will support all tests, leaving the vast majority of possiblehypotheses unexamined” (CP 6.530).

To illustrate the potential effects that hypothesis testing can have onother projects in a well-designed experimental program, Peirce likens itto the game of twenty questions. I think of some object or other in theuniverse and play the role of experience. You can test up to twenty

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hypotheses by asking strategic questions like “Is what you are thinkingof a living thing?” to which I respond “yes” or “no”: “If the questioningis skillful, the object will invariably be guessed . . . . twenty skillfulhypotheses will ascertain what two hundred thousand stupid onesmight fail to do” (CP 7.220).13 As in the game of twenty questions, itcan make sense to test a well-chosen or informative hypothesis first,even if it is very unlikely to be true: “[t]he hypothesis to be taken up isnot necessarily a probable one. The cuneiform inscriptions could neverhave been deciphered if very unlikely hypotheses had not been tried”(CP 8.251).

One of Peirce’s favorite examples of this strategy in action was thedeciphering of cuneiform:

In the first steps that were made toward the reading of cuneiforminscriptions, it was necessary to take up hypotheses which nobodycould have expected would turn out to be true,—for no hypothesispositively likely to be true could be made. But they had to be provi-sionally adopted,—yes, and clung to with some degree of tenacitytoo,—as long as the fact did not absolutely refute them. For that wasthe system by which in the long run such problems would quickestfind their solutions. (NEM4, 184; see also CP 7.220)

3.3 Abduction and Peirce’s Larger Picture of Research MethodologyI have argued that both of the traditional interpretations leave out cru-cial aspects of Peirce’s distinctive views on abduction. The GenerativeInterpretation focuses on a stage of inquiry at which it is too early toemploy retroductive reasoning: how could economic considerationssupply a recipe for dreaming up hypotheses? The Justificatory Interpre-tation points misleadingly to a later stage, long after scientists havemade the difficult decisions about which hypotheses were worth test-ing. In contrast, according to the Pursuitworthiness Interpretation,abductive reasoning makes practically grounded comparative recom-mendations about which available hypotheses are to be tested. Peircedirects our attention to a stage of scientific inquiry that comes after theinitial discovery or proposal of hypotheses but prior to the kind ofexperimental testing that would be needed to make epistemic evalua-tions of any real importance; to that period of scientific practice aftertheories are hatched but before our ugly ducklings have matured intoswans. As part of Peirce’s systematic attempts to think about the quali-ties that factor into decisions about whether investigating an idea lookspromising or seems worthwhile, he shifts the focus away from epis-temic appraisal, placing the emphasis instead on how “economic” orpractical considerations legitimately enter in to judgments aboutwhether a theory is worth investigating and developing further.

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Peirce himself acknowledged that his reflections about abductionwere often not “as evident as would be desirable,” but nevertheless heinsisted that abduction is a distinct mode of inference (CP 7.97–7.98;see also NEM4, 183–184). Perhaps these three readings are not incom-patible and Peirce’s sustained reflection on abduction—which spannedmore than fifty years—paints an overall picture of scientific methodol-ogy into which each of these aspects can fit.14 At the very least, I sub-mit, that the Pursuitworthiness Interpretation will have to be part ofany attempt come to grips with Peirce’s wide ranging remarks aboutabduction.

The Pursuitworthiness Interpretation should also be of interest to con-temporary philosophers of science. Not only does it highlight the need fora reevaluation of the relationship between Peirce’s abduction and con-temporary discussions of inference to the best explanation, on this read-ing Peirce also opens up a new set of questions about the methodology ofscience and the cognitive structure of reasoning important to scientificinquiry. Consideration of the role of pursuitworthiness judgments in sci-entific practice could provide a fresh perspective or yield insights bearingon decision-making processes in public policy and on the question: “Canwe theorize a role for value judgments and practical considerations in the-ory choice without compromising the objectivity of science?”

But how well does Peirce’s account of abduction, as glossed by thePursuitworthiness Interpretation, map on to judgments that scientistsactually make? Scientists do make judgments about pursuitworthinessand sometimes we can catch them in the act. Let us close with an invivo glance at some pursuitworthiness judgments that physicists aremaking with respect to current research programs in quantum gravity.We find Lee Smolin making rather complex judgments regarding thepursuitworthiness of string theory:

String theory succeeds at enough things so that it is reasonable to hopethat parts of it, or perhaps something like it, might comprise somefuture theory. But there is also compelling evidence that somethinghas gone wrong. . . .

So, is string theory still worth studying, or should it be declared afailure, as some suggest? The fact that many hopes have been disap-pointed and many key conjectures remain unproved may well be rea-son enough for some to give up working on string theory. But they are notreasons for the research to stop altogether.

What if sometime in the future someone found a way to formulatea string theory that led uniquely to the standard model of particlephysics, was background-independent, and lived only in the three-dimensional nonsupersymmetric world we observe. Even if theprospects for finding such a theory seem slim, it is a possibility—underscoring the general wisdom that a diversity of research programs ishealthy for science, a point we’ll return to later.

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So string theory is certainly among the directions that deservemore investigation. But should it continue to be regarded as the dom-inant paradigm of theoretical physics? Should most of the resourcesaimed at the solution of the key problems in theoretical physics con-tinue to support research in string theory? Should other approachescontinue to be starved in favor of string theory? Should only stringtheorists be eligible for the most prestigious jobs and research fellow-ships, as is now the case? I think the answer to all these questionsmust be no. String theory has not been successful enough on any level tojustify putting nearly all our eggs in one basket. (Lee Smolin 2006, 198–199; emphasis added)

Similar judgments are on display in the following passage fromCarlo Rovelli:

The tentative theories we have, such as loop gravity, strings or non-commutative geometry, are courageous attempts that are worth pur-suing but badly incomplete. Loop gravity has no ambition of being atheory of everything: it is just a background-independent theory ofquantum space-time. In string theory, a background independentformulation seems as far away as ever. More crucially, none of thesespeculative theories has received any empirical support from experi-ment. Worse, phenomena such as proton decay, supersymmetric par-ticles and signs of extra dimensions were predicted, but haven’t shownup. Rarely have we been so far from a theory of everything. Thinkingthat we might be close to it is the common error of those who mistaketheir own expectations for the ultimate truth. (Rovelli 2005, 259a)

Rovelli here effortlessly combines the judgment that many of these the-ories are worth pursuing with the epistemically qualified judgmentsthat they are also are speculative, incomplete, uncorroborated empiri-cally, and a long way from the final story. While the project of getting areally clear picture of cognitive structure of the various judgments thatscientists make at different stages of inquiry must be left for anotheroccasion, here we see pursuitworthiness judgments playing an impor-tant role in scientific practice in ways that fit fairly well with the rea-soning processes Peirce describes.15

University of Notre Dame DMckaug564@aol.com

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NOTES

*Abbreviated versions of this essay were presented to the Charles S. PeirceSociety at the December 2007 Eastern Division Meeting of the American Philo-sophical Association in Baltimore, MD and to the philosophy department atBoston College on February 18, 2008. I would like gratefully to thank bothgroups for the useful feedback they provided on each occasion.

1. For the purposes of this paper I employ a distinction between “epistemic”(considerations that bear on the truth or falsity of a hypothesis) and “non-epistemic” factors. For example, logical consistency and fit with the empirical dataare relevant to the epistemic evaluation of Einstein’s general relativity, whereas theracial prejudices that some physicists harbored against Einstein are not. Keep inmind that there are further controversies about whether this distinction is sharpand about whether particular considerations such as simplicity are epistemicallyrelevant desiderata or merely pragmatic. An anonymous referee astutely pointedout that a distinction between belief and action becomes especially blurry on prag-matist accounts of belief as a habit of action. However, these difficulties do notappear to challenge my criticisms of the Justificatory Interpretation or the centralpoint of the paper. If it turned out that either that there is no useful distinction tobe drawn here, this would make my question “Have we been asking the rightquestions about theory appraisal?” and the task of characterizing the cognitivestructure of these judgments all the more pressing.

2. There were many ways of drawing the D-J distinction and the contrastclasses were not always entirely clear (see Hoyningen-Huene 1987; 2006). Butwhether the D-J distinction was understood conceptually, say, (i) as a contrastbased on distinctive goals appropriate to each context, (ii) as a contrast between adescription of scientific practice and a systematic normative treatment of methodsfor testing and evidential support, (iii) as a contrast between matters of logic andthose of psychology, or (iv) as a distinction between processes appropriate to earlyand late stages of the temporal development of a research tradition, “discovery”inevitably served as a dust bin for neglected questions.

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3. The most striking support for this reading that I have been able to findcomes from Peirce’s lecture notes for a course he had planned to teach. There hedescribes abduction or retroduction as,

that process in which the mind goes over all the facts the case, absorbs them, digeststhem, sleeps over them, assimilates them, dreams of them, and finally is promptedto deliver them in a form, which, if it adds something to them, does so only becausethe addition serves to render intelligible what without it, is unintelligible (LectureI of a planned course, MS 857: 4–5, n.d.).

This comes from an undated and unpublished first lecture of a course Peirce hadplanned to teach. The reference is to the Peirce manuscripts, but the relevant pas-sage is also available online at the Commens Dictionary of Peirce’s Terms, edited byMats Bergman & Sami Paavola under the entry on “retroduction” athttp://www.helsinki.fi/science/commens/dictionary.html.

4. This should give us pause, but it isn’t a knock down refutation: there is, forexample, at least one place—in “A Neglected Argument for the Reality of God”(1908)—where Peirce seems to equate retroduction with “the spontaneous con-jectures of instinctive reason” (CP 6.475). However, as Kapitan (2000) points out,there are a number of passages where Peirce is quite insistent that inferencesinvolve consciously judging premises to provide support for a conclusion:

Peirce described an inference as “the conscious and controlled adoption of a beliefas a consequence of other knowledge” (CP 2.442, 2.144, 5.109), which “consists inthe thought that the inferred conclusion is true because in any analogous case ananalogous conclusion would be true” (CP 5.130). Its aim is “to find out, from theconsideration of matters and things already known, something else that we had notknown before” (MS 628:4), thus, to increase our grasp of truth. The occurrences ofthe phrases “as a consequence of” and “because” in these passages not only indicatea premise-conclusion relation, but carry a causal significance since inference “pro-duces” or “creates” a belief in the mind of the reasoned (CP 2.148).

5. Compare with Peirce’s discussion of apagogue in Book II, Chapter XXV ofAristotle’s Prior Analytics, which he translates as “abduction” or, in his view moreprecisely, as “retroduction” in Peirce’s Cambridge Lectures on “Reasoning and theLogic of Things” (1898).

6. Despite all the attention to inference to the best explanation in the contem-porary literature, it remains remarkably difficult to state exactly how this ‘infer-ence’ is supposed to go. There is also disagreement about whether, once the sloganis spelled out, the inference is any good and how important it is for science (seevan Fraassen 1989). My point here is that I am not at all sure these authors aretalking about the same thing as Peirce.

7. For additional criticism of the identification of abduction and inference tothe best explanation see: Hintikka, 1998.

8. We can gain a bit more distance from the standard interpretations of Peirceand sense for the ongoing relevance of his thought to contemporary discussions,by situating Peirce’s discussion of abduction with respect to a trend since the mid-1970s to see a third context of inquiry—“contexts of pursuit”—located on a spec-trum between the extremes of discovery and justification (see: Achinstein 1970,1980, 1987, 1990, 1993; Franklin 1993a, 1993b; Larry Laudan 1977, 1980,1981, 1996; Rachel Laudan 1986; McKinney 1993, 1995; Nickles 1980; Whitt1985, 1990). Franklin succinctly summarizes the idea as follows:

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I add my own support for a tripartite classification scheme of scientific activity: dis-covery, pursuit, and justification. By discovery I mean the process by which a the-ory or hypothesis is generated and proposed. Pursuit is the further investigation ofa theory or of an experimental result. Justification is the decision process by whichthe scientific community comes to accept or reject a theory or an experimentalresult as part of the corpus of scientific knowledge (Franklin 1993a, 253).

9. For a helpful chronological review of some of the most important placeswhere Peirce discusses abduction, including Peirce’s 1885 Harvard Lectures, 1866Lowell Lectures, Popular Science Monthly series (1878), “A Theory of ProbableInference” (1883), Cambridge Lectures (1898), as well as papers from 1901–1903and 1910, see: Niiniluoto, 1999.

10. Kapitan’s primary focus, however, is on whether abduction is anautonomous mode of reasoning as Peirce claimed and Kapitan argues that it isreducible to a combination of deductive and inductive inferences. I have no stakein the autonomy thesis or its denial.

11. In his more cautious moments, Peirce does allow—as any efficient methodought to allow—that, if our background information strongly suggests that one ofthe potential explanations is likely to be true, this can serve as a reason for placingit among the hypotheses to be pursued: “On the other hand, if one of the admis-sible hypotheses presents a marked probability of the nature of an objective fact, itmay in the long run promote economy to give it an early trial” (CP 6.534).

12. More precisely: “Now economy, in general, depends upon three kinds offactors: cost, the value of the thing proposed, in itself; and its effect upon otherprojects” (CP 7.220). In the same section, Peirce elaborates:

Under the head of cost, if a hypothesis can be put to the test of experiment withvery little expense of any kind, that should be regarded as a recommendation forgiving it precedence in the inductive procedure. For even if it is barely admissiblefor other reasons, still it may clear the ground to have disposed of it. . . . Under thehead of value, we must place those considerations which tend toward an expecta-tion that the given hypothesis may be true. . . . The third category of factors ofeconomy, those arising from the relation of what is proposed to other projects, isespecially important in abduction, because very rarely can we positively expect agiven hypothesis to prove entirely satisfactory; and we must always consider whatwill happen when the hypothesis breaks down (CP 7.220).

13. As Peirce puts it elsewhere:

Let us suppose that there are thirty-two different possible ways of explaining a setof phenomena. Then, thirty-one hypotheses must be rejected. The most economi-cal procedure, where it is practicable, will be to find some observable fact which,under conditions easily brought about, would result from sixteen of the hypothe-ses and not from any of the other sixteen. Such an experiment, if it can be devised,at once halves the number of hypotheses. . . . When such an experiment, or any-thing approaching such an experiment, is possible, it is clear that it is unwise toadopt any other course. (CP 6.529)

14. Consider where each of the three readings—inspired suggestion, practicalpursuitworthiness judgment, and epistemic evaluation—locates abductive reason-ing in the following passage:

The scientific man finds himself confronted by phenomena which he seeks to gen-eralize or to explain. His first attempts to do this, though they will be suggested bythe phenomena, can yet, after all, be reckoned but mere conjectures; albeit, unless

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there be something like inspiration in them, he never could make a successful step.Of those conjectures,—to make a long matter short,—he selects one to be tested. In hischoice, he ought to be governed solely by considerations of economy. If, for exam-ple, the prospect is that a good many hypotheses to account for any one set of facts,will probably have to be taken up and rejected in succession, and if it so happensthat, among these hypotheses, one that is unlikely to be true can probably be disposed ofby a single easy experiment, it may be excellent economy to begin by taking up that. Inthis part of his work, the scientist can learn something from the businessman’s wis-dom. At last, however, a hypothesis will have been provisionally adopted, on proba-tion; and now, the effort ought to be to search out the most unlikely necessaryconsequence of it that can be thought of, and that is among those that are readilycapable of being brought to the test of experiment. The experiment is made. If theprediction from the hypothesis fails, its failure may be so utter as to be conclusive;or, maybe, nothing more than an alteration of the defective theory needs to beundertaken. (Peirce 1901d, 73; emphasis added)

15. For a more extended discussion about the descriptive adequacy of theclaim that pursuitworthiness judgments do, in fact, play an important role in sci-entific practice see McKaughan 2007. In addition to quantum gravity, I considerpursuitworthiness judgments as they arise in case studies on the Bohr and Ruther-ford model of the atom (Achinstein 1993), Maxwell and the kinetic theory ofgases (Achinstein 1987, 1990), episodes of controversial science—including thealleged discovery of ‘polywater’ in 1962 and of cold fusion in 1989—(McKinney1993; 1995), conceptual problems in Newton’s rejection of Cartesian theory andquest for a successor (Whitt 1990), the Daltonian vs. “affinitist” atomic traditionsin the nineteenth century (Whitt 1990), and in the events treated in Franklin(1993).

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