Language, Truth, and Logic

Language, Truth and Logic is a 1936 book about meaning by the philosopher Alfred Jules Ayer, in which the author defines, explains, and argues for the verification principle of logical positivism, sometimes referred to as the criterion of significance or criterion of meaning. Ayer explains how the principle of verifiability may be applied to the problems of philosophy. Language, Truth and Logic brought some of the ideas of the Vienna Circle and the logical empiricists to the attention of the English-speaking world.

According to Ayer's autobiographical book, Part of My Life, it was work he started in the summer and autumn of 1933 that eventually led to Language, Truth and Logic, specifically Demonstration of the Impossibility of Metaphysics—later published in Mind under the editorship of G.E. Moore. The title of the book was taken ("To some extent plagiarized" according to Ayer) from Friedrich Waismann's Logik, Sprache, Philosophie.

According to Ayer, analytic statements are tautologies. A tautology is a statement that is necessarily true, true by definition, and true under any conditions. A tautology is a repetition of the meaning of a statement, using different words or symbols. According to Ayer, the statements of logic and mathematics are tautologies. Tautologies are true by definition, and thus their validity does not depend on empirical testing.

Synthetic statements, or empirical propositions, assert or deny something about the real world. The validity of synthetic statements is not established merely by the definition of the words or symbols they contain. According to Ayer, if a statement expresses an empirical proposition, then the validity of the proposition is established by its empirical verifiability.

Propositions are statements that have conditions under which they can be verified. By the verification principle, meaningful statements have conditions under which their validity can be affirmed or denied.

Statements that are not meaningful cannot be expressed as propositions. Every verifiable proposition is meaningful, although it may be either true or false. Every proposition asserts or denies something, and thus is either true or false.

Ayer distinguishes between ‘strong’ and ‘weak’ verification, noting that there is a limit to how conclusively a proposition can be verified. ‘Strong’ (fully conclusive) verification is not possible for any empirical proposition, because the validity of any proposition always depends upon further experience. ‘Weak’ (probable) verification, on the other hand, is possible for any empirical proposition.

Ayer also distinguishes between practical and theoretical verifiability. Propositions for which we do not have a practical means of verification may still be meaningful if we can verify them in principle.