1. Descartes claims, using the example of the chiliogon (a thousand-sided polygon), that there are some objects he can understand but not imagine, and other objects he can both understand and imagine. The only explanation he can think of for the difference is that in the case of the latter, but not the former, aspects of his thought are caused from outside his mind. This, he says, makes the existence of corporeal bodies `probable.’
2. Descartes claims to have a clear distinct idea of the objects of the understanding and of the imagination as separate. Since the mind is an object of the understanding but not the imagination, and the body is a typical object of the imagination, the mind and the body are separate. He then repeats his standard error yet again: Since he can conceive of them as separate, it must be the case that they could exist separately. So logical and ontological dependence are run together once more.
3. If these contents from without didn’t come from bodies, than God would be deceiving him. But God is not a deceiver. Therefore, he concludes, the contents of his imagination must come from bodies.
4. Descartes has no sound basis for the first premise above. His earlier arguments yielded (fallaciously) only the conclusion that God wouldn’t deceive him about the contents of clear and distinct ideas. He hasn’t shown that God has designed him so as to make no general errors. But he has given no argument showing that he has a clear and distinct idea that images of bodies are caused by bodies.
5. Descartes supplements the above argument with another. He experiences his mind, he says, as a unity, whereas his body comes in infinitely divisible parts. This confirms his view that they are fundamentally different kinds of things, but this point is just an aside. His mind, being a unity, unifies his various and disparate perceptions. This enables him to compare them for accuracy. Thus he can distinguish truth from error and wakeful perception from dreaming if he tries. And thus the last of his original doubts is dispelled.
6. So we have the Cartesian philosophy. Its central claims are:
(i) Truly reliable knowledge is and can be established by applying the pure understanding to abstract ideas. That is, true (certain) knowledge is achieved by use of mathematics and logic. It does not result from sensory perception, though sensory perception can be useful if it is judged carefully in light of the understanding.
(ii) The world divides fundamentally into two parts: matter, which takes up space and is governed by mechanics, and mind, which takes up no space and is governed by reason.
Most non-scientists in Western cultures probably still believe (ii) but not (i). Scientists don’t believe either (i) or (ii), for reasons we’ll now explore in the work of our three remaining philosophers, Hume, Russell, and Dennett.