Tay asked:

Does Rene Descartes provide a persuasive reply to the challenge of scepticism? Why or why not? If not, how is it (or is it?) possible to know anything for certain?

Answer by Graham Hackett

Arguably Descartes provides a persuasive answer to the sceptical challenge. For him, resisting the sceptical challenge comes from knowing with certainty, that our experiences do not come from a vivid dream, or from an evil demon capable of contriving and controlling our perceptual inputs, or a matrix-type scenario , where we are brains in vats, fed a rich, but unreal set of experiences electronically. However we represent the sceptical scenario, you can easily get the picture; how can we be sure that our experiences are not real knowledge, but the result of one of these situations?

Descartes proposes that whatever the real situation, he knows that he thinks, and no dream scenario, or evil demon can persuade him otherwise. If he thinks, then he has the foundation for asserting the truth of his experiences. Thus our introspection persuades us that we exist. Descartes uses the expression ‘clear and distinct’ to explain his certainty. My beliefs about my mental states have this special status for me because of:

1. Infallibility. If I believe P (where P is a proposition about my mental states) , and am justified in the belief, then P must be true. So provided I believe something, I cannot be mistaken about it. It is logically impossible that I would be wrong in a belief about my mental states. In fact, stated strongly, the belief guarantees justification, truth, and therefore knowledge about our mental states.

2. Omniscience. For any proposition P which is true, it is impossible that I would fail to know P. Here, truth and justification guarantee knowledge.

3. Indubitability. If I believe P, it is logically impossible for anyone else to doubt that I know P. Belief guarantees justification and truth of P

4. Incorrigibility. For any proposition P which I believe this guarantees knowledge, plus the impossibility of anyone being able to show that I am wrong.

5. Truth-Sufficiency. For any true proposition P about my mental states it is logically impossible that I would not be justified in knowing P. A bit like omniscience, except that here, truth guarantees justification.

6. Self Warrant. A bit stronger – If I believe something about my mental states, then that is sufficient to guarantee justification and truth, and hence knowledge.

I recommend reading a paper by William Alston ‘Varieties of Privileged Access’,
American Philosophical Quarterly, Vol. 8, Jul., 1971). for a full account.

This is a persuasive account, but it is possible to raise doubts. Wittgenstein argued that if words get their meaning by referring to my experience, then what I mean by ‘experience’ means ‘my experience’. It is impossible that anyone else could have my experiences. But that means that it makes no sense to think of other people having experience – because ‘experience’ refers to my experience alone. Wittgenstein argued that a ‘private’ language of this kind is logically impossible. Wittgenstein concludes that we cannot fix the meaning of words by appealing to private sensations. Instead, we have to use something public, available to other people.

This matters for the Cartesian stand against scepticism, since it casts doubts on Descartes claims of infallibility, omniscience, indubitability etc, for introspection.

So is it possible to know anything with certainty, if we hold that Descartes’ ‘clear and distinct’ perception is not enough? We may have to abandon two notions about knowledge if we are to make any headway with this question.

Firstly, we may have to abandon the absolute element in knowledge. To say that something is flat in an absolute sense may be impossible, because there will always be microscopic irregularities in any surface. Likewise, in order to say that we know anything in the same absolute way, we would have to be able to rule out every possible alternative, even the most remote ones, (such as an evil demon, or a vivid dream). It is impossible to do this, which is why the sceptical argument has such traction, and can rob us of our ability to claim knowledge. Do we have to do this? Do we have to rule out every possible alternative before we can say we know something? There is a whole area of epistemology based on qualifying the use of the word ‘know’ with respect to the context in which we use the term, and the alternative propositions which can be judged relevant or irrelevant to our considerations. Look up ‘contextualism’ and ‘relevant alternatives’ in any good dictionary of philosophy.

Secondly, we may have to accept that we are unable to pursue any philosophic enquiry to the limits without accepting some bedrock unquestionable principles. G.E Moore may have had this idea in mind when he made his famous proof of the existence of external objects;

‘Here is one hand, here is another. They are two physical objects, so there must be an external world’. (Read ‘Proof of an External World’ Moore, – freely available on the internet).

Moore offers this as a perfectly sound proof, with distinct premise and conclusion. You do not always have to prove your premises (e.g.; here are two hands) before you develop an argument, otherwise you would never begin to get off the ground at all. Wittgenstein, who was interested in this proof by Moore developed these ideas further in his posthumously published work ‘On Certainty’. To use his own expression, there are certain underlying principles which are like the hinges of a door; without these hinges, the door will not either open or close. Similarly, without the ‘hinges’ of knowledge, it is not possible to conduct any philosophic enquiry. The sceptics would not even be able to argue for their own position if it was never possible to claim knowledge. Scepticism might even be said to be self defeating.