The problem of mental causation is essentially coeval with the mind–body problem. Descartes arguably invented the latter when, in Meditation 2, he asked ‘But what then am I?’ to which he replied ‘A thing which thinks’, and then went on to argue, in Meditation 6, that ‘it is certain that this I is entirely and absolutely distinct from my body, and can exist without it’. As every student of western philosophy knows, Descartes's view was that minds and bodies constitute two disjoint categories of substance: minds are immaterial substances whose essential nature is thinking, while bodies are material substances located in physical space whose essence consists in being extended in space. Presumably, substance dualism of this form was not startling news to anyone at the time. However, Descartes, alone among the great rationalists of his day, urged a further view: minds and bodies are in causal interaction with each other, minds influencing bodies in voluntary actions and bodies influencing minds in perception and sensation.