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A. J. Ayer’s Language, Truth, and Logic was the first systematic presentation in English of the doctrines of logical positivism. While a student at Oxford University, Ayer spent the academic year 1932-1933 in Vienna attending the lectures and meetings of a group of philosophers, mathematicians, and scientists who called themselves the Vienna Circle. He quickly absorbed the basic tenets of the Vienna Circle and wrote Language, Truth, and Logic shortly after his return to England.

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Ayer accepts the traditional empiricist view of philosophers such as David Hume that all genuine propositions either are analytic (in the sense, for Ayer, of being true solely by virtue of linguistic rules) or are empirically verifiable. Since analytic propositions are true in virtue of the meanings of the words composing them, they cannot be used to make factual assertions about the world. The tautological proposition “Either it is raining or it is not” is analytic in this sense, and it therefore says nothing about actual weather conditions. Because analytic propositions are true by virtue of meaning alone, Ayer believes, they can be known independently of experience; that is, they are knowable a priori.

In addition to tautologies and conceptual truths (for example, “Red is a color”), according to Ayer, the set of analytic propositions includes the necessarily true propositions of logic and mathematics. Traditional empiricists and Ayer maintain that propositions that are not analytic, and so are not true solely by virtue of linguistic rules, cannot be known a priori; they cannot be known by pure reason alone. It requires perceptual experience or some sort of empirical investigation to determine whether such propositions are true or false. Because these propositions are not true solely in virtue of linguistic rules, they have factual content and thus can be used to make informative assertions about the way things are in world.

Traditional empiricism was thus primarily a claim about knowledge and justification: The only propositions that can be known without empirical investigation are ones that are true by definition (for Ayer and other logical positivists, by virtue of linguistic rules), so any informative proposition can be known only on the basis of perceptual experience. In other words, the empiricists denied the rationalist claim that a priori knowledge of the world is possible.

With the principle of verification, Ayer and the logical positivists take empiricism to the semantic level by focusing on meaning rather than knowledge. This principle says that, in order to be meaningful, a proposition that is not analytic must be empirically verifiable. It must be possible, that is, to specify what sort of perceptual experiences would demonstrate that the principle is true or false. Propositions that do not satisfy this criterion of meaningfulness are declared by Ayer and the logical positivists to be meaningless and devoid of cognitive significance. Ayer’s version of verificationism does not require that the proposition be conclusively verifiable. His version more modestly requires only that there be possible sensory experiences that are relevant to the question of whether a statement is true or false. Furthermore, in Ayer’s version of the principle of verification, it is not necessary for verification to be practically possible; it is enough that verification could be carried out in principle.

With the principle of verification in place, Ayer goes on to argue that much of traditional metaphysical philosophy is not merely false but also meaningless nonsense. Metaphysics, as traditionally conceived, attempts to determine the nature of the world in itself, the reality “behind” the phenomenal world given in perceptual experience. The metaphysician, using pure a priori reasoning, claims to penetrate the veil of appearance and to arrive at the...

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