 Rationally credible or warranted or justified belief, Lewis thought, is probable on the evidence, but the presentation of his views on probability was underdeveloped in MWO, and complex, and sometimes confusing, in AKV. In AKV, Lewis defended an a priori account of probability or what he sometimes called expectation. However, he rejected the Principle of Indifference often associated with a priori accounts, understood as the principle that in the absence of any reason for thinking one a priori possibility more likely than another, they are equiprobable (Lewis 1946, 306–314). The expectation or probability \(a/b\) of a proposition \(P\) is always relative to some set of empirical data or premises \(D\). The expectation corresponds to an a priori valid estimate of the frequency of some property mentioned in \(P\) in some reference class mentioned in \(P\) , which estimate is derived from data or premises \(D\), given the a priori valid principles of probabilistic inference, including the principles of induction. Hypothetical or conditional probability statements that are a priori valid license valid probabilistic inferences from premises about evidence or data to probabilistic conclusions. However, for Lewis, both hypothetical and categorical probability are always relativized to an evidence base, despite his occasional apparent talk of a priori valid probability statements as licensing inferences from empirical evidence to a (detachable) conclusion “Probably, P”.