Born at Champtercier in Provence on January 22, 1592, Gassendi is best known today as the author of the notoriously hostile Fifth Objections to Descartes’ Meditations. He was also a priest, an astronomer, a humanist scholar, and a defender of Galileanism. But the achievement that earned him a reputation on par with Descartes’ among their contemporaries was the construction of a neo-Epicurean, atomist system of philosophy.

Gassendi spent his adult life in Paris and Provence, traveling back and forth between the two regularly. As a young man, he briefly held the chair of philosophy at the Université d'Aix. In 1645 he was made professor of mathematics at the prestigious Collège Royal. (This appointment was due to his reputation as an astronomer, since astronomy was a branch of mixed mathematics. However, for most of his career, he was dependent on a succession of increasingly prestigious patrons. His chief intellectual relationships were with the other members of the Mersenne circle and especially with Mersenne himself.)

Gassendi died leaving his major work, the Syntagma philosophicum, unfinished. His executors assembled the text we have from manuscripts produced over decades; this may help to explain the somewhat repetitive and disorganized nature of the almost two-thousand-folio-page Syntagma. It covers what Gassendi, following the Hellenistic philosophers, conceived of as philosophy's three parts: logic, physics, and ethics. In logic – which, for Gassendi, encompasses epistemology, psychology, and methodology – he insists upon the senses as the source of ideas and the criterion of truth. In physics – which includes metaphysics and all of the natural sciences – he argues that the world is composed of atoms with size, shape, and motion (or motive power), moving in absolute space and time. He is thus a key founder of the mechanical philosophy, although it is worth noting that in practice Gassendian physics is highly nonreductionist and almost entirely qualitative. Gassendi's ethics forms a key part of his reconciliation of Epicureanism with Catholicism, while at the same time being heavily influenced by Hobbes.