What the Split Cost

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.