  Although, unlike most of his fashionable contemporaries and immediate successors, Descartes was not an atomist, he was, like the others, a mechanist about the properties of matter. Bodies are machines that work according to their own laws. Except where there are minds interfering with it, matter proceeds deterministically, in its own right. Where there are minds requiring to influence bodies, they must work by ‘pulling levers’ in a piece of machinery that already has its own laws of operation. This raises the question of where those ‘levers’ are in the body. Descartes opted for the pineal gland, mainly because it is not duplicated on both sides of the brain, so it is a candidate for having a unique, unifying function.