  To establish that atoms are the primary principles or elements of matter, Gassendi draws deeply on the well of ancient atomist argument. This sort of ‘historical evidence’ or ‘testimony’ tolerably counts as one manner of support for physical claims at the dawn of the early modern period. Given Gassendi's pronounced empiricism, though, the degree of his reliance on such testimony is surprising. He strikes an even more surprising anti-empiricist chord in the introductory chapter of his Physics (Physicae Prooemium), pondering a stipulative account of the natural world that consists of the right physical postulates. In the end, though, this approach is unacceptable for Gassendi, as such a stipulative physics raises more questions than it answers. His alternative tack is a series of reasoned defenses of basic atomist tenets—frequently, amplifications and restatements of Epicurean and Lucretian arguments—including claims as to the existence and status of atoms as the primary material principles.