Descartes’ intuition and deduction thesis claims to find intuitions such as that we exist and then use them as the premises in deductive arguments to ultimately prove God’s existence and the existence of the external world.
The meaning of ‘intuition’ and ‘deduction’ and the distinction between them
Descartes thinks we can gain informative (synthetic) knowledge through a priori means by intuition and deduction. Intuition is when the rational mind apprehends the truth or falsity of something with immediacy, which means without any process of reasoning or inference. The mind simply ‘grasps’ the rational rules by which we intuit that 2+2 necessarily equals 4. Descartes claims there is a ‘natural light’ of the mind which makes us recognize certain truths because we cannot doubt them.
Deduction is using premises to reach a conclusion the truth of which is entailed by the truth of the premises. If we can know that the premises are true and that the conclusion follows deductively from them then we can know the truth of the conclusion.
René Descartes’ notion of ‘clear and distinct ideas’
Descartes claims that the cogito is apprehended by his mind in a special way which he calls clarity and distinctness. A clear idea is ‘present and accessible to the attentive mind’ – analogous to perceiving something visual clearly with our eye. An idea is distinct when it is so sharply separated from all other ideas that every part of it is clear’. Descartes claims that since the Cogito is a clear and distinct idea which he knows to be true, then clarity and distinctness must ‘as a general rule’ be a sign of truth.
Descartes’ cogito as an example of an a priori intuition
Descartes’ points out that we cannot doubt our own existence since that presupposes that we exist in order to do the doubting. We can therefore see that our existence is a clear and distinct idea intuited a priori.
Empiricist responses to the cogito
Hume vs Descartes on personal identity. Hume is an empiricist and thinks we can only derive knowledge from experience. Hume points out that the experience from which we could derive the idea of an enduring mental substance would have to be an experience of something constant and invariable. However, Hume claims we never experience any such thing within our own mind.
Whenever we try to introspect and experience this ‘self’ we believe in, we only ever experience particular mental states such as sensations or emotions. We never experience a constant and invariable ‘self’ which supposedly exists as the experiencer of those mental states. All we really experience ourselves to be is a “bundle” of ever-changing mental states.
When we think about the contents of our own mind, we never experience an enduring mental substance which could be the ‘I’ of the cogito. There is no persisting substance over time that could constitute an enduring personal identity.
Descartes’ replies to this kind of argument by claiming it is clear and distinct that thoughts require a thinker.
Descartes’ arguments for the existence of God
Deductive arguments using intuitions.
After the Cogiro, Descartes’ admits that while we can know that clear and distinct ideas are true when we are apprehending them, it’s surely possible that they become untrue when we are not, perhaps because of God or an evil demon deceiving us. Therefore he feels the need to prove that is not the case. To do that, Descartes first attempts to prove the existence of God with his cosmological, ontological and trademark arguments.
The trademark argument
P1. I exist (intuition)
P2. I have an idea of a supremely perfect being in my mind. (intuition)
P3. The causal adequacy principle is true. (intuition).
C1. I am not causally adequate to create the idea of a supremely perfect being, only a supremely perfect being is. (deduction.
C2. A supremely perfect being exists (deduction)
Only God is causally adequate for the idea of God in my mind.
Descartes argues that the ‘natural light’ yields the causal adequacy principle – that there must be as much reality in the cause as there is in its effect. Just as something cannot come from nothing, neither can what is more perfect come from what is less perfect.
He argues we imperfect beings are causally inadequate to create the perfect concept of God, therefore God must exist as the only causally adequate explanation of our concept of God.
Empiricist responses to the trademark argument
Hume argues that the concept of God is not innate but can be created by our minds. We start by imagining finite human qualities like goodness and imagine what they were like without limit by abstractly negating finitude/imperfection to create the concept not-finite/not-perfect, which is the concept infinite/perfect. We then combine goodness and infinite/perfect to imagine God’s omnibenevolence, and so too with God’s other attributes.
Descartes responds to this kind of argument by claiming that recognizing something as imperfect presupposes that you have the idea of perfect. That goes against the order of Hume’s claim however, that we deduced perfection from imperfection.
Descartes’ proof of the external world
Deductive argument using intuitions.
P1. I have a clear and distinct idea of a physical substance (as something which has extension and is changeable).
P2. My perceptions are involuntary and thus cannot come from my own mind over which I have voluntary control.
C1. Therefore, there are three other options for the origin of my perceptions: physical objects, God, or a tendency to have false beliefs that I cannot correct.
P3. The source of these perceptions cannot be God, nor would God create me with a strong tendency to believe something false which I cannot correct. In either case, God would be deceiving me, yet it is a clear and distinct idea that deception involves fault or imperfection, and God is perfect.
P4. According to Descartes’ other arguments, God exists.
C2. Therefore, through process of elimination, the source of my perceptions must be physical objects, and therefore, an external world exists.
Empiricist responses to Descartes’ cogito
Hume vs Descartes on personal identity. Hume is an empiricist and thinks we can only derive knowledge from experience. Hume points out that the experience from which we could derive the idea of an enduring mental substance would have to be an experience of something constant and invariable. However, Hume claims we never experience any such thing within our own mind.
Whenever we try to introspect and experience this ‘self’ we believe in, we only ever experience particular mental states such as sensations or emotions. We never experience a constant and invariable ‘self’ which supposedly exists as the experiencer of those mental states. All we really experience ourselves to be is a “bundle” of ever-changing mental states.
When we think about the contents of our own mind, we never experience an enduring mental substance which could be the ‘I’ of the cogito. There is no persisting substance over time that could constitute an enduring personal identity.
Descartes’ replies to this kind of argument by claiming it is clear and distinct that thoughts require a thinker.
Arguably Descartes is just influenced by the common sense ordinary experience of attributing thoughts to thinkers, but if he was truly doubting everything then why not the supposed link between thoughts and thinkers also?
Empiricist responses to Descartes’ arguments for the existence of God
Hume argues that the concept of God is not innate but can be created by our minds. We start by imagining finite human qualities like goodness and imagine what they were like without limit by abstractly negating finitude/imperfection to create the concept not-finite/not-perfect, which is the concept infinite/perfect. We then combine goodness and infinite/perfect to imagine God’s omnibenevolence, and so too with God’s other attributes.
Descartes responds to this kind of argument by claiming that recognizing something as imperfect presupposes that you have the idea of perfect. That goes against the order of Hume’s claim however, that we deduced perfection from imperfection.
Arguably the origin of the concept of perfection is merely a subjective preference for order over chaos since that typically enables survival which we have evolved to desire. It’s hard to see what objective basis there could be for perfection, but in that case the concept of God is only subjectively perfect which would not then place any constraints on its causal adequacy.
Empiricist responses to Descartes’ proof of the external world
Hume’s fork. Hume claims that ‘all objects of human reason or inquiry’ are either ‘relations of ideas’ or matters of fact.
A priori reasoning can only tell us about the relations between ideas, i.e. analytic knowledge (true by definition). E.g. “a bachelor is an unmarried man”.
A posteriori reasoning can only tell us about matters of fact, i.e. synthetic knowledge (true by the way the world is). E.g. “The sun will rise tomorrow”.
Truths about the relations of ideas are not dependent on ‘what is anywhere existent in the universe’. Their truth is therefore established a priori as a matter of definition, regardless of what is true of the universe. E.g., 1+1=2 regardless of whatever matters of fact are true. So, analytic truths are different from synthetic truths and arrived at by different types of reasoning, hence the reason for Hume’s fork.
Analytic truths cannot be denied without contradiction, since there is no possibility of it changing to be false since it does not depend on anything which changes (the universe).
The intuition and deduction thesis uses a priori reasoning yet comes to a conclusion about matters of fact regarding the existence of an external world.
Integration: According to Hume’s fork Descartes’ conclusion cannot be justifiably known from the premises. Intuitions of the relations of ideas and deductions made about them therefore cannot be about the world. A priori intuition and deduction only provide us with analytic knowledge of the relations of ideas, not synthetic matters of fact.