Language, Truth and Logicis one of Ayer's most important books and may be considered as one of the most influential philosophical works of the 20th century. In the second edition (1946), Ayer clarified some of his ideas and replied to his critics, but essentially his philosophical position remained the same. He called his philosophy "logical empiricism," a variation of logical positivism, the philosophical orientation he learned in Vienna. He was largely influenced by the thought of the 20th century philosophers Bertrand Russell and Ludwig Wittgenstein and by the earlier empiricism of George Berkeley and David Hume.

The book is a milestone in the development of philosophical thought in the 20th century. The implications of Ayer's "logical empiricism" would be felt by many branches of the discipline of philosophy, especially metaphysics, ethics, and philosophy of religion, and also logic, mathematics, and the philosophy of science. Although Ayer acknowledged the influences upon his philosophical perspective, he remained an independent thinker, accepting no position uncritically.

Ayer asserted that the criterion of meaning is found in the "verification principle": "We say that a sentence is factually significant to any given person if, and only if, he knows how to verify the proposition which it purports to express—that is, if he knows what observations would lead him, under certain conditions, to accept the proposition as being true, or reject it as being false." (Language, Truth and Logic). The a priori statements of logic and mathematics do not claim to provide factual content. Those statements can be said to be true only because of the conventions which govern the use of the symbols that make up the statements.

Ayer was known in the 20th century for his rigorous attack on metaphysics and as the main representative of the British empirical tradition. Genuine statements are either logical or empirical. Metaphysical statements do not purport to express either logical truths or empirical hypotheses. For that reason, metaphysical statements are pseudo-statements and do not have any meaning. The metaphysician had been tied to the attempt to construct a deductive system of the universe from "first principles." These first principles, Ayer argued, can never be derived from experience. They are merely hypotheses. As a priori principles, they are hypotheses only, and therefore are tautologies and notcertain empirical knowledge.

Theology, as a special branch of metaphysics which attempts to gain knowledge that transcends the limits of experience (for example, the affirmation of the existence of God) is not only false but it too has no meaning. Value statements in ethics and aesthetics are also meaningless, not genuine statements, and can be understood as emotive utterances of an imperative character.

Ayer therefore discovered for philosophy a function in the 20th century. Once the traditional tasks of philosophy have been discarded, philosophy can be seen as an intellectual discipline which endeavors to clarify the problems of science. Philosophy is, therefore, finally identical with the logic of science.

In Language, Truth and Logic Ayer argued that it is the task of the philosopher to give a correct definition of material things in terms of sensation. The philosopher does not deal with the properties of things in the world, but only with the way we speak of them. The propositions of philosophy are not factual, but linguistic in character: " (Propositions) … do not describe the behavior of physical, or even mental, objects; they express definitions, or the factual consequences of definitions."

In the second edition of Language, Truth and Logic, Ayer provided an extended reworking of his notion of the verification principle. It was this principle which was chiefly criticized by the philosophical commentators. It would seem that the verification principle, as formulated by Ayer, is a kind of meaningless metaphysical statement that the verification principle itself was supposed to prohibit.

In his later works, Ayer proceeded boldly, and with wisdom and clarity, to deal with the major problems that have confronted and confounded other 20th century philosophers: such problems as perception, induction, knowledge, meaning, truth, value theory, other minds, the mind-body dichotomy, personal identity, and intention. Ayer was always an original and bold thinker who, in later life, espoused a more selective assessment of metaphysics due to the works of his trusted colleagues. His views on death, dying and the afterlife were slightly altered after briefly dying for four minutes and subsequently being revived. His death on June 27, 1989 marked the end of the second golden age of British philosophy.

Ayer provided an autobiographical volume which is filled with trenchant philosophical insights about the role of the philosopher in the 20th century; A. J. Ayer, Part of My Life: The Memoirs of a Philosopher (1977). Ayer was a popular broadcaster for the British Broadcasting Company (BBC). One of the most exciting broadcasts was in the form of a debate with a Jesuit Christian philosopher; see "Logical Positivism—A Debate" delivered on the BBC June 13, 1949, with A. J. Ayer and F. C. Copleston, published in P. Edwards and A. Pap (editors), A Modern Introduction to Philosophy (1957). Among the many commentators of A. J. Ayer's philosophical perspective, the following are helpful: Carl G. Hempel, Aspects of Scientific Explanation (1965); Viktor Kraft, The Vienna Circle: The Origin of Neo-Positivism (1969); John Wisdom, "Note on the New Edition of Professor Ayer's Language, Truth and Logic," reprinted in Wisdom's Philosophy and Pyscho-analysis (Oxford, 1953); H. H. Price, "Critical Notice of A. J. Ayer's The Foundation of Empirical Knowledge," in Mind (1941); H. H. Price, "Discussion: Professor Ayer's Essays," in Philosophical Quarterly (1955); D. J. O'Connor, "Some Consequences of Professor A. J. Ayer's Verification Principle," in Analysis (1949-1950); W. V. O. Quine, "Two Dogmas of Empiricism," in From a Logical Point of View (1953).

Reflections on Ayer's legacy can be found in "The Logical End of an Empire, " Economist (July 8, 1989) and "Logic in High Gear, " Spectator (July 8, 1989).