  The empiricist basis of Ayer’s attitude to meaning was laid first in his reading of Hume. The thought that no idea had any empirical significance unless it was suitably related to an impression stayed with him, and was reinforced both by his reading of Wittgenstein’s Tractatus and by the time spent in Vienna with the Logical Positivists. His first formulation of a criterion of meaning, the principle of verification, was in the first edition of LTL (1936), where he claimed that all propositions were analytic (true in virtue of their meaning) or else either strongly verifiable or weakly verifiable. Strong verification required that the truth of a proposition be conclusively ascertainable; weak verification required only that an observation statement be deducible from the proposition together with other, auxiliary, propositions, provided that the observation statement was not deducible from these auxiliaries alone. This rapidly proved defective: any proposition P conjoined with ‘if P then O’, where ‘O’ is an observation statement, will yield O, without this being deducible from ‘if P then O’ alone. So in the second edition Ayer amended the principle to read: a statement is directly verifiable if it is either an observation statement or is such that an observation statement is derivable from it in conjunction with another observation statement (or observation statements), such derivability not being possible from the conjoined observation statement(s) alone. And a statement is indirectly verifiable if, first, in conjunction with certain other premises it entails one or more directly verifiable statements that are not derivable from these other premises alone, and, second, that these other premises “do not include any statement that is not either analytic, or directly verifiable, or capable of being independently established as indirectly verifiable.” (LTL 2nd ed. P. 17).