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.

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.

The title of the book was taken ("To some extent plagiarized" according to Ayer) from Friedrich Waismann's Logik, Sprache, Philosophie.

Ayer distinguishes between ‘strong’ and ‘weak’ verification, noting that there is a limit to how conclusively a proposition can be verified.

Ayer rejects the metaphysical thesis that philosophy can give us knowledge of a transcendent reality.

He argues that metaphysical statements have no literal meaning, and that they cannot be subjected to criteria of truth or falsehood.

A significant consequence of abandoning metaphysics as a concern of philosophy is a rejection of the view that the function of philosophy is to propose basic principles of meaning and to construct a deductive system by offering the consequences of these principles of meaning as a complete picture of reality.

According to Ayer, no proposition concerning "matters of fact" can ever be shown to be necessarily true, because there is always a possibility that it may be refuted by further empirical testing.

Ayer agrees with, and elaborates on, Kant's explanation of the distinction between analytic and synthetic judgments.

Analytic observations give us new knowledge, because they reveal unsuspected implications of our statements and beliefs.

For Ayer, ethical or aesthetic judgments are subjective rather than objective, and cannot be demonstrated to be true or false.

Ayer's logical empiricism makes an important contribution to philosophy in that it provides a method of putting an end to otherwise irresolvable philosophical disputes.

In Ayer's logical empiricism, philosophy is no longer seen as a metaphysical concern, it is not a search for first principles nor an attempt to provide speculative truths about the nature of ultimate reality.

The principle of verifiability, however, may become a means to arbitrarily reject any abstract or transcendent concept, such as “truth,” “justice,” or "virtue."

Ayer is careful to explain that the verification principle is a definition of meaning, and that it is not an empirical proposition.

The most frequently expressed reservation about the principle is whether it is itself verifiable; this was addressed in the fictional dialogue "Logical Positivism: a discussion".

Fifty years after he wrote his book, he said: "Logical Positivism died a long time ago.