  The need for a theory of empirical knowledge in Gassendi's view stems from the inadequacy of past and contemporary responses to the Skeptics, whose puzzles he finds particularly compelling in the form of the tropes as presented by Sextus Empiricus. The Skeptical tropes call our attention to the general problem of whether, given the fallibility of the senses, any knowledge is possible and in what the warrant for such would consist. Gassendi turns the problem around, limiting its scope to finding warrant for empirical knowledge and declaring the special solution to resolve the general case. His significant move in this regard is to propose, against the Stoics (and, as we have seen, Descartes), that it is not a necessary condition of our knowing some claim that we be certain of it. This probabilism immediately broadens the range of what we can know through the senses—as does his suggestion that we include among such claims to knowledge those assertions about what may be hidden to the senses yet legitimately inferred from evidence of the perceptually given. This is his theory of sign-based inference, which suggests such inferences are legitimate just in case they would be agreed to by a great enough number of experts giving testimony—or else would be false only on pain of contradiction.