 Autrecourt’s Second Letter seeks to temper this skepticism, but only to a degree. According to him, the only things of which we can be certain are the principle of non-contradiction (i.e., “nothing both is and is not”) and other propositions that can be derived from this principle. He maintains a causal, proto-Humean skepticism about existential inferences: “From the fact that some thing is known to be, it cannot be inferred evidently, by evidentness reduced to the first principle, or to the certitude of the first principle, that there is some other thing” (Second Letter 11). As he continues, he says that the only substance of which we can possess evident knowledge is his own soul