Rene Descartes

Descartes wanted a foundation that could not be shaken, so he set out to reject anything open to the least doubt, and pushed the doubt as far as it would go: the senses deceive, dreams are indistinguishable from waking while they last, and even mathematics could be the work of a powerful deceiver bending the mind at every step. What survives is that the one being deceived must exist in order to be deceived. From that single point the Meditations builds outward, and the first thing it builds is God.

There are two arguments and they are different. The first observes that a finite mind contains the idea of something infinite and perfect, and holds that such an idea cannot have been produced by the mind that contains it, so its cause must be what it represents. The second, in the fifth meditation, argues from the concept of a perfect being to its existence in the manner of the older ontological proof. Both are load bearing, because the reliability of clear and distinct perception is what the whole reconstruction rests on and God is what guarantees it. Contemporaries objected at once that this is circular, since the proofs themselves are known clearly and distinctly, and the objection has never been fully disposed of. He dedicated the work to the theology faculty of the Sorbonne and meant it to serve them.

The division between thinking substance and extended substance was meant to protect the soul, and it did the opposite of what he expected. Once matter is defined as extension and nothing else, the physical world has no purposes in it, and every explanation of how a thing behaves becomes a question of size, shape and motion. That is an enormous gain for physics and a permanent problem for everything else. Animals, having no thinking substance, become mechanisms. The human body becomes a machine with a passenger, and the relation between the two has to be located somewhere, which is why he ended up proposing the pineal gland as the point of contact, an answer nobody has ever found satisfying. The mind and body problem in its modern form starts here.

The church was not reassured. His works were placed on the Index in 1663, thirteen years after his death. There is a further detail his admirers find awkward and the atlas records because he recorded it: he traced his whole programme to three dreams on a single night in November 1619, which he took to have been sent, and in response to which he vowed a pilgrimage. That account survives through an early biographer who worked from a notebook that has since been lost, so its wording cannot now be checked against the original.