  One of the greatest challenges to Gassendi's atomism from an internal perspective is the seemingly quixotic nature of his search for empirical foundations for atomism. In his era there is not the barest hint of perceptually-derived knowledge of anything so small. There is, he thinks, an empirically viable source of at least some claims concerning atoms—the indirect data of indicative signs. Yet the general tenor of his characterization and defense of physical atomism signals a departure from his customary empiricism. As in his defense of an indivisibility thesis, Gassendi relies foremost on reason, not experience, to account for the origin and quantity of atoms and what he takes to be their essential and inessential properties, their internal impetus, motion, and causal role, and their contribution to the motions and qualities we attribute to macrophysical objects. He is hardly to be faulted in this regard given the lack of direct empirical evidence available. Moreover, his responses to ancient and early modern critics of atomism are firmly within their strategic bounds, as they typically rely on reasoned argument without appeal to empirical force.