In Language, Truth, and Logic, Sir A. J. Ayer presents a modified version of logical positivism that he prefers to call “logical empiricism.” However, the doctrines, particularly their implications for philosophy, are largely those of logical positivism, and the work serves to bring these together succinctly and vigorously. Therefore, the book has had great importance as a positivistic document and as a center of controversy about positivistic tenets. In it, Ayer offers to solve the problems of reality, perception, induction, knowledge, meaning, truth, value, and other minds. He presents no great new idea; rather, he has modified and brought into logical consistency solutions proposed by others. In the introduction to the 1946 second edition, Ayer provided further explication and modified a few beliefs, but essentially his position remained unchanged.
The Verification Principle
Ayer attacks metaphysics, saying that he will deduce the fruitlessness of attempting knowledge that transcends the limits of experience from the “rule which determines the literal significance of language.” The sentences of metaphysics, failing to meet this rule, are meaningless.
Ayer finds the criterion of meaning 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.” Another possible kind of meaningful sentence is the tautology. However, any sentence that is neither a tautology nor a verifiable proposition (by this criterion) is a mere pseudoproposition, a meaningless sentence.
Certain provisions qualify this tenet. Ayer distinguished practical verifiability and verifiability in principle. Some sentences are not practically verifiable because of inconvenience or the present state of science and culture. If one knows what observations would decide such a matter if one were in a position to make them, the proposition is verifiable in principle. A further distinction is that between “strong” verifiability and “weak” verifiability. According to the “strong” theory, advanced by the Vienna Circle of logical positivists, a sentence is meaningful only if it is conclusively verifiable empirically; according to the “weak” theory, it is meaningful if experience may render it probable. Ayer chooses the “weak” theory on the basis that because no empirical demonstration is ever 100 percent conclusive, the “strong” theory leaves no empirical statement meaningful. By using the “weak” theory, Ayer believes he allows general propositions of science and propositions about the past, which previous positivistic writers found problematic, to have meaning. The proposed principle rules out such assertions as the statement that the world of sense is unreal and such questions as whether reality is one substance or many. No experience could decide these issues, so they have no literal significance. Metaphysicians have usually been misled by the grammar of language so that they posit an entity (“substance,” “Being”) where grammar requires a noun as the subject of a sentence, even though thought may exert no such requirement.
By the abandonment of metaphysics, the philosopher is freed from the function of constructing a deductive system of the universe from first principles, for first principles cannot come from experience, whose propositions are only hypotheses and never certain. However, if they are taken a priori, they are only tautologies, which cannot apply to the universe as factual knowledge.
The problem of induction can be set aside as unreal. It is the attempt to prove that certain empirical generalizations derived from past experience will hold good also in the future. It must have either an a priori or an empirical solution. However, in the first case, it is improper to apply...
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