  The core of Gassendi's mature philosophy and natural philosophy is his atomist matter theory. He initially borrows on historical atomist sources—primarily Epicurus—but quickly integrates a range of atomist claims into his proposed empirically defensible complex of physical, metaphysical, and ethical perspectives. Some aspects of his atomism fit poorly with the mechanical philosophy it is supposed to exemplify. Yet Popkin is correct in suggesting that Gassendi's atomism is a premier instance of his philosophical and scientific pursuits constituting one and the same project. The putative empirical support does not stand up to close inspection, and his particular brand of this matter theory is inconsistent with other elements of the mechanical philosophy.  Nonetheless, his atomism provides an innovative and consistent model for micro-level accounts of macro-level physical structures and their behavior, throughout all manner of natural, psychological, and social domains. In this last regard, Gassendi's views resemble those of his acquaintance, Hobbes, though with greater attention to core physical details of the matter theory, and fine-grained notions as to how the micro-mechanisms may operate.