  The starting point of Gassendi's philosophy, in Popkin's view, is skepticism about knowledge of essences, mitigated by allowance for warranted beliefs about appearances and causal knowledge to which we are entitled just because it helps us to make sense of beliefs about appearances. Among the intriguing elements of Popkin's assessment is the suggestion that Gassendi promotes an early prototype of inference to the best explanation (IBE), defending atomism by appealing to its explanatory value. Gassendi's strategy is a bit more complicated, for he cannot grant that the evidence could be equally compelling for all competing theses, given his view that atomism is the physical thesis which best makes intelligible our experiential data to begin with. The underlying methodological suggestion is that, in considering among physical theses, the way we understand and interpret correlative data about appearances may depend on which such thesis we are entertaining. In calling attention to Gassendi's appeal to IBE, Popkin identifies a principal challenge in binding together principal themes of Gassendi's corpus, how to be an atomist and a thoroughgoing empiricist at once (Fisher 2005).