  A further element of Gassendi's scientific method consists in his appeals to indirect evidence for claims about the unobservable, and his willingness to count such evidence as adequate empirical grounds for maintaining viable hypotheses. The most prominent instance of this strategy is his embrace of an atomist matter theory as a ‘most likely hypothesis’. Of necessity, any evidence on behalf of this hypothesis is to be found in the indirect form of ‘indicative’ signs—these are the surface level phenomena for which he takes the existence of atoms to be a sine qua non condition. A paramount instance of such evidence in his view is what we may see through the microscope, when the visual phenomena are explicable only given the existence of the micro-structures we perceive—and in turn those are explicable only in terms of yet smaller structures. Gassendi maintains that the microscopic observations of crystalline formation and dissolution demonstrate the molecular structure of matter, a key aspect of his atomism. Equally innovative is his appeal to this putative source of warrant in support of his matter theory.