  Hume was an influential figure in the formation of Ayer’s philosophical views, so it is no surprise to find Ayer’s approach to inductive inference modeled on Hume’s. Ayer defined inductive inference in negative terms, as involving all factual inference in which the premises did not entail the conclusion. All such inferences, Ayer claimed, presumed the uniformity of nature, an assumption he put in terms of assuming that the future will, in relevant respects, resemble the past (1956, p. 72). To unambiguously cover cases of retrodiction, the assumption is better put in terms of the unobserved resembling, in relevant respects, the observed. Ayer agreed with Hume that relying on any ‘principle’ of the uniformity of nature was not going to help justify inductive inference, given that such a principle was itself not demonstrable. A similar argument applied to any other principles that may have been thought to supply the missing ingredient, such as an appeal to universal causality, or to laws of nature. These were also not demonstrably true, so would require justification themselves, and any appeal to these principles in such a justification would be viciously circular.