In the English-speaking world Pierre Gassendi is probably best known as the author of a set of Objections to Descartes’s Meditations. These Objections, the fifth of seven sets collected by Mersenne, are relatively long and full, and suggestive of a number of distinctively Gassendist doctrines-for example his nominalism, his insistence on distinguishing mathematical objects from physical ones, and his doubt whether we can know the natures of things, even our selves. Perhaps more prominent than these doctrines, however, is a kind of materialism. Gassendi adopts the ironic form of address ‘O Mind’ in challenging Descartes’s claim that one’s nature has nothing to do with body. He insists that all ideas have their source in the senses, and he sketches an account of perception that dispenses with a role for a pure intellect but emphasizes the contribution of the brain. In physics he was partial to explanation in terms of the motions of matter, ultimately the motions of material atoms. These points suggest that Gassendi was a mechanistic materialist of some kind, and they link him in intellectual history with Hobbes, who proposed that physical as well as psychological phenomena were nothing more than motions in different kinds of body.