Gassendi Pierre (1592–1655), French philosopher and scientist who advocated a via media to scientific knowledge about the empirically observable material world that avoids both the dogmatism of Cartesians, who claimed to have certain knowledge, and the skepticism of Montaigne and Charron, who doubted that we have knowledge about anything. Gassendi presented Epicurean atomism as a model for explaining how bodies are structured and interact. He advanced a hypothetico-deductive method by proposing that experiments should be used to test mechanistic hypotheses. Like the ancient Pyrrhonian Skeptics, he did not challenge the immediate reports of our senses; but unlike them he argued that while we cannot have knowledge of the inner essences of things, we can develop a reliable science of the world of appearances. In this he exemplified the mitigated skepticism of modern science that is always open to revision on the basis of empirical evidence. Gassendi’s first book, Exercitationes Paradoxicae Adversis Aristoteleos (1624), is an attack on Aristotle. He is best known as the author of the fifth set of objections to Descartes’s Meditations (1641), in which Gassendi proposed that even clear and distinct ideas may represent no objects outside our minds, a possibility that Descartes called the objection of objections, but dismissed as destructive of all reason. Gassendi’s Syntagma Philosophiae Epicuri (1649) contains his development of Epicurean philosophy and science. His elaboration of the mechanistic atomic model and his advocacy of experimental testing of hypotheses were crucially important in the rise of modern science. Gassendi’s career as a Catholic priest, Epicurean atomist, mitigated skeptic, and mechanistic scientist presents a puzzle – as do the careers of several other philosopher-priests in the seventeenth century – concerning his true beliefs. On the one hand, he professed faith and set aside Christian doctrine as not open to challenge. On the other hand, he utilized an arsenal of skeptical arguments that was beginning to undermine and would eventually destroy the rational foundations of the church. Gassendi thus appears to be of a type almost unknown today, a thinker indifferent to the apparent discrepancy between his belief in Christian doctrine and his advocacy of materialist science. See also DESCARTES, EPICUREANISM , SKEP- TIC. R.A.W.