 When Leibniz insists that every truth or fact requires a sufficient reason, what does he mean by “sufficient reason?” In some texts he suggests that sufficient reason is an “a priori proof”. This should not be understood in Kantian terms as a proof that doesn’t require any input from sense experience. Rather, Leibniz uses the term “a priori” in its original pre-Kantian meaning, which means an argument from causes to effects. An a priori proof is a proof that reflects the causal order. Thus a sufficient reason would be a proof that is both a demonstration and an explanation (see Adams 1994: 109).