Its central thesis is that any genuine proposition must be either a tautology or empirically verifiable. In Ayer’s terminology, a proposition is said to be a tautology if and only if it is ‘analytic’, and it is said to be analytic if and only if ‘its validity depends solely on the definitions of the symbols it contains’ (LTL, 78). Ayer’s wording here is a bit loose; for, strictly speaking, it is sentences, not the propositions they express, which contain symbols. But I shall leave the discussion of this point till the next section. Because their validity (i.e. their truth) depends only on their meaning, and not on how things are ‘in the world’, Ayer holds that analytic propositions are devoid of factual content: they make no claim about matters of fact. Consequently, he sometimes abbreviates the central thesis to the bare assertion that any factual proposition must be empirically verifiable.