Rene Descartes1596—1650

Some dates

1543: publication of Copernicus’s 1543: publication of Copernicus’s De De RevolutionibusRevolutionibus

1633: Galileo arrested1633: Galileo arrested1641: publication of the 1641: publication of the MeditationsMeditations

Some reasons to doubt the senses

Past errorPast error Insanity/delusionInsanity/delusionDreamsDreamsPossibility of demon/evil genius/malicious Possibility of demon/evil genius/malicious

godgod

How do these lead us to doubt the senses?

1.1. All our knowledge of the external world is All our knowledge of the external world is based on sense experiencebased on sense experience

2.2. Unless I have a reason to think my sense Unless I have a reason to think my sense experience is reliable, I have no reason experience is reliable, I have no reason to believe in an external worldto believe in an external world

3.3. I can’t have any reason to think my I can’t have any reason to think my senses are reliablesenses are reliable

4.4. Therefore, I have no reason to believe in Therefore, I have no reason to believe in an external worldan external world

What can I know?

I think; therefore I am!

What am I?

Plato:(428-347BCE)

I am a featherless biped

Aristotle:(384-322BCE)

I am a rational animal

I am a thinking thing!

Descartes:

Personal Identity

What am I?

Descartes:

I am my mindI am my mind I am essentially rational, only accidentally I am essentially rational, only accidentally

an animalan animalThe demon The demon thought experimentthought experiment indicates indicates

that I could exist even if my body did not, that I could exist even if my body did not, so I am not my bodyso I am not my body

Metaphysics and Epistemology

Metaphysics:Metaphysics: the study of what there is, the study of what there is, the fundamental nature of realitythe fundamental nature of reality

Epistemology:Epistemology: theory of knowledge and theory of knowledge and justified belief, reason, evidence, etc.justified belief, reason, evidence, etc.

Rationalism

All or most of our knowledge is All or most of our knowledge is a prioria priori, , i.e., knowable independently of i.e., knowable independently of experience, i.e., on the basis of pure experience, i.e., on the basis of pure reasonreasonEpistemological viewEpistemological view

All or most of our concepts are innate All or most of our concepts are innate Psychological view (aka nativism)Psychological view (aka nativism)

Empiricism

All or most of our knowledge is All or most of our knowledge is a a posterioriposteriori, i.e., knowable only on the basis , i.e., knowable only on the basis of experienceof experienceEpistemological viewEpistemological view

All or most of our concepts are learned All or most of our concepts are learned Psychological view (sometimes called Psychological view (sometimes called

“concept empiricism”)“concept empiricism”)

Descartes’s skepticism

Gives us reason to doubt the sensesGives us reason to doubt the senses

Highlights distinction between appearance Highlights distinction between appearance and realityand reality

1. Nothing is ever directly present to the mind 1. Nothing is ever directly present to the mind except its own ideas (e.g., sense experiences)except its own ideas (e.g., sense experiences)

2. To know anything about the external world, I 2. To know anything about the external world, I would first have to know these ideas accurately would first have to know these ideas accurately represent the external worldrepresent the external world

3. But I couldn’t have any non-question-begging 3. But I couldn’t have any non-question-begging reason for believing that, thus can’t know itreason for believing that, thus can’t know it

4. Therefore, I can’t know anything about the 4. Therefore, I can’t know anything about the external worldexternal world

1. Nothing is ever directly present to the mind 1. Nothing is ever directly present to the mind except its own ideas (e.g., sense experiences)except its own ideas (e.g., sense experiences)

2. To know anything about the external world, I 2. To know anything about the external world, I would first have to know these ideas accurately would first have to know these ideas accurately represent the external worldrepresent the external world

3. But I couldn’t have any non-question-begging reason for believing that, thus can’t know it

4. Therefore, I can’t know anything about the external world

If I can come up with an If I can come up with an a prioria priori argument argument for the reliability of sense-perception, it for the reliability of sense-perception, it won’t be question-beggingwon’t be question-begging

I think; therefore I am!

What do I know?

That I thinkThat I thinkThat I existThat I existThat I’m thinking about ….That I’m thinking about ….

E.g., that I’m in pain (understood as purely E.g., that I’m in pain (understood as purely mental), that I’m having a visual experience mental), that I’m having a visual experience as of a desk, etc.as of a desk, etc.

That I am a thinking thing (that I am my That I am a thinking thing (that I am my mind)mind)

Mind is better known than body, even Mind is better known than body, even though bodies are perceptible, minds are though bodies are perceptible, minds are notnot

Surprising, if we confuse understanding with Surprising, if we confuse understanding with imaginingimagining

Imagination:Imagination: capacity for forming sensory capacity for forming sensory images of things (e.g., seeing “in the images of things (e.g., seeing “in the mind’s eye”)mind’s eye”)

Understanding:Understanding: capacity for pure, capacity for pure, nonsensory thoughtnonsensory thought

triangle

(-2, -4), (3, 7), (1, -5)

Not all knowledge involves imaginationNot all knowledge involves imagination (In fact, none does)(In fact, none does)

Why does imagination have such a grip on Why does imagination have such a grip on us?us?

Because it gives us clear---though not Because it gives us clear---though not distinct---ideasdistinct---ideas

Clear:Clear: accessible to the attentive mind (opposed to accessible to the attentive mind (opposed to obscure)obscure)

Distinct:Distinct: sharply separated from other ideas sharply separated from other ideas (opposed to confused)(opposed to confused)

Wax example Piece of wax has a certain taste, odor, size,

color, shape, texture, solidity, sound. Bring it near a heat source and all of these

change. Still we know that it is the same piece of wax.

Our knowledge of the wax is not through the senses but through the intellect alone.

To perceive the wax is not strictly to see it, but to understand it The wax itself is an invisible substance that has

perceptible properties. We can’t imagine the wax itself, though we can understand it

What do I know?

That That I know thatI know that I think I think That That I know thatI know that I exist I exist

So I know something about knowledgeSo I know something about knowledge

What does my perception of my own existence What does my perception of my own existence have that qualifies it for being knowledge? have that qualifies it for being knowledge?

That I clearly and distinctly perceive it to be trueThat I clearly and distinctly perceive it to be true

Therefore, Therefore, whatever I clearly and distinctly whatever I clearly and distinctly perceive to be true, really is trueperceive to be true, really is true

Clarity and Distinctness Principle

Whatever I clearly and distinctly perceive to Whatever I clearly and distinctly perceive to be true, really is truebe true, really is true

Can we give a more rigorous proof?Can we give a more rigorous proof?

1.1. God exists.God exists.2.2. God wouldn’t let me be deceived when I’m doing my God wouldn’t let me be deceived when I’m doing my

very best to avoid errorvery best to avoid error3.3. The best I can do is assent to only what I clearly and The best I can do is assent to only what I clearly and

distinctly perceive to be truedistinctly perceive to be true4.4. Therefore, whatever I clearly and distinctly perceive to Therefore, whatever I clearly and distinctly perceive to

be true, really is truebe true, really is true

Argument for God’s Existence---Background

Formal realityFormal reality/existence: existence in fact, /existence: existence in fact, real existencereal existence

Objective realityObjective reality/existence: existence in /existence: existence in thoughtthought

Argument for God’s Existence:Causal principles known a priori

Everything has a cause.Everything has a cause.

Some corollaries:Some corollaries: There must be at least as much reality in the There must be at least as much reality in the

cause as in the effect. cause as in the effect. (otherwise, (otherwise, partpart of the effect would be uncaused) of the effect would be uncaused)

There must be at least as much formal reality There must be at least as much formal reality in the cause as there is objective reality in the in the cause as there is objective reality in the effecteffect

(all causes must have formal reality)(all causes must have formal reality)

Argument for God’s Existence:

1.1. I have an idea of God, i.e., an infinite beingI have an idea of God, i.e., an infinite being

2.2. This idea has infinite objective realityThis idea has infinite objective reality

3.3. There is as at least as much formal reality in There is as at least as much formal reality in the cause as objective reality in the effectthe cause as objective reality in the effect

4.4. Therefore, this idea is caused by a thing with Therefore, this idea is caused by a thing with infinite formal realityinfinite formal reality

5.5. Therefore, an infinite thing existsTherefore, an infinite thing exists

6.6. I.e., God existsI.e., God exists

Argument for reliability of cognitive faculties:

1.1. God (exists and) is infinite (from earlier argument)God (exists and) is infinite (from earlier argument)2.2. Therefore, God is perfect (from 1)Therefore, God is perfect (from 1)3.3. To deceive is to be lacking in some perfectionTo deceive is to be lacking in some perfection4.4. Therefore, God is not a deceiver (2, 3)Therefore, God is not a deceiver (2, 3)5.5. God is (directly or indirectly) responsible for my God is (directly or indirectly) responsible for my

existenceexistence6.6. If God allowed me to be mistaken, even when I’m doing If God allowed me to be mistaken, even when I’m doing

my very best to avoid error, he would be a deceiver (5)my very best to avoid error, he would be a deceiver (5)7.7. Since he’s not a deceiver (4), it must be that when I’m Since he’s not a deceiver (4), it must be that when I’m

doing my very best, I can’t go wrong (4, 6)doing my very best, I can’t go wrong (4, 6)8.8. The best I can do is to assent to only what I clearly and The best I can do is to assent to only what I clearly and

distinctly perceive to be truedistinctly perceive to be true9.9. Therefore, what I clearly and distinctly perceive to be Therefore, what I clearly and distinctly perceive to be

true really is true (Clarity and Distinctness Principle)true really is true (Clarity and Distinctness Principle)

Problem: how is it that we ever err?

By not using judgment properlyBy not using judgment properly

Judgment is really result of two faculties: Judgment is really result of two faculties: Intellect:Intellect: perceives ideas perceives ideas Will:Will: chooses what to believe chooses what to believe

Intellect is finite, will is infiniteIntellect is finite, will is infinite Since the will extends farther than the intellect, it Since the will extends farther than the intellect, it

is possible to believe what we don’t perceive to is possible to believe what we don’t perceive to be true.be true.

The essence of material things

Clear and distinct ideas of extension (taking Clear and distinct ideas of extension (taking up space), motion, durationup space), motion, duration

Innate knowledge of numbers figures, Innate knowledge of numbers figures, movements, etc.movements, etc.

The principles of geometry are clear and The principles of geometry are clear and distinct, thus knowable.distinct, thus knowable.

Mathematics in general more certain than Mathematics in general more certain than belief based on sensationbelief based on sensation

Descartes’s epistemology

IntuitionIntuition: self-evident knowledge; knowledge that : self-evident knowledge; knowledge that does not depend on other knowledge (basic does not depend on other knowledge (basic beliefs)beliefs)

DemonstrationDemonstration: knowledge that does depend on : knowledge that does depend on other knowledge (nonbasic beliefs)other knowledge (nonbasic beliefs)

Foundationalism:Foundationalism: some beliefs are basic some beliefs are basic (noninferentially justified, foundational); and all (noninferentially justified, foundational); and all other justified beliefs ultimately derive their other justified beliefs ultimately derive their justification from inferential connections to basic justification from inferential connections to basic beliefsbeliefs

Proof: string together intuitions, so that each step Proof: string together intuitions, so that each step in the proof is self-evident, can render something in the proof is self-evident, can render something clear and distinct that wasn’t previouslyclear and distinct that wasn’t previously

Clarity and distinctness goes away over time, Clarity and distinctness goes away over time, becomes possible to doubtbecomes possible to doubt

Existence of God, fact that he is no deceiver, Existence of God, fact that he is no deceiver, ensures that what is – or was once – clearly and ensures that what is – or was once – clearly and distinctly perceived is truedistinctly perceived is true

Knowledge of God’s existence essential for Knowledge of God’s existence essential for perfect knowledge on the basis of proofperfect knowledge on the basis of proof

The Cartesian Circle

Famous objection to Descartes’ project in Meditations:Famous objection to Descartes’ project in Meditations:

Descartes assumes at beginning of 3rd Meditation that Descartes assumes at beginning of 3rd Meditation that what is clearly and distinctly perceived is truewhat is clearly and distinctly perceived is true

Uses this to prove God’s existenceUses this to prove God’s existence Uses this to prove that what is clearly and distinctly Uses this to prove that what is clearly and distinctly

perceived is true.perceived is true.

Problem: there’s a vicious circularity here: can’t know that Problem: there’s a vicious circularity here: can’t know that God exists unless we God exists unless we alreadyalready know that what we clearly know that what we clearly and distinctly perceive is trueand distinctly perceive is true

Partial solution? In proofs, clarity and distinctness goes away over time, In proofs, clarity and distinctness goes away over time,

becomes possible to doubtbecomes possible to doubt

Existence of God, fact that he is no deceiver, ensures Existence of God, fact that he is no deceiver, ensures that what is – or was once – clearly and distinctly that what is – or was once – clearly and distinctly perceived is trueperceived is true

Knowledge of God’s existence essential for “perfect Knowledge of God’s existence essential for “perfect knowledge” on the basis of remembered proofknowledge” on the basis of remembered proof

Atheist geometer can have a sort of knowledge, but not the Atheist geometer can have a sort of knowledge, but not the unshakeable knowledge that a theist can haveunshakeable knowledge that a theist can have

Return of the External World:I. My Own Body

Recall imagination/understanding distinctionRecall imagination/understanding distinctionProbabilistic argument for the existence of bodyProbabilistic argument for the existence of body

1.1. I could exist/endure without capacity for I could exist/endure without capacity for imaginationimagination

2.2. Therefore, the imagination depends on Therefore, the imagination depends on something distinct from mesomething distinct from me

3.3. A body conjoined to me could be the means by A body conjoined to me could be the means by which I imagine thingswhich I imagine things

4.4. No other candidates seem very likelyNo other candidates seem very likely5.5. Therefore, I probably have a bodyTherefore, I probably have a body

Return of the External World:II. Other Bodies

1.1. The capacity for sensing that is in me is The capacity for sensing that is in me is passive (I don’t choose to create ideas of passive (I don’t choose to create ideas of sensation)sensation)

2.2. I’m not the cause of my ideas of sense – I’m not the cause of my ideas of sense – they’re typically involuntarythey’re typically involuntary

3.3. There must be some source of these ideas, There must be some source of these ideas, outside of meoutside of me

4.4. If the real cause is anything other than the If the real cause is anything other than the bodies the ideas are of, then God is a bodies the ideas are of, then God is a deceiver. deceiver.

5.5. God is not a deceiver, so there must be bodiesGod is not a deceiver, so there must be bodies

Return of the External World:III. The Nature of Bodies

Sensation only tells me that there are bodies, gives me Sensation only tells me that there are bodies, gives me clear and distinct ideas of bodies as objects of clear and distinct ideas of bodies as objects of mathematics.mathematics.

Anything else I believe is my choice, not God’s doingAnything else I believe is my choice, not God’s doing

““Bodies as objects of mathematics”Bodies as objects of mathematics” i.e., bodies insofar as they have a particular shape, size, i.e., bodies insofar as they have a particular shape, size,

motion, solidity, etc.motion, solidity, etc. ““Primary Qualities”Primary Qualities”

But not insofar as they have particular taste, odor, color, But not insofar as they have particular taste, odor, color, sound, etc.sound, etc. ““Secondary Qualities”Secondary Qualities”