A.J. Ayer, answered from the texts and cited to the page.
The traditional disputes of philosophers are, for the most part, as unwarranted as they are unfruitful.1 That blunt verdict from Language, Truth and Logic — my 1936 statement of the logical-positivist programme — is where the elimination of metaphysics begins. The core claim is this: a statement is literally meaningful if and only if it is either analytic (a tautology, true in virtue of the meanings of its terms, telling us nothing about the world) or empirically verifiable in principle (such that some possible observation would be relevant to determining its truth or falsehood).2
Statements that satisfy neither condition are not false — they are not even in the running to be false. They are pseudo-statements: sequences of words that wear the grammatical costume of genuine assertions while expressing nothing at all.3 Metaphysics, on this analysis, is the systematic production of pseudo-statements. There are two ways the trick is performed.
Either a metaphysical sentence contains a word that is erroneously believed to have meaning — a word for which no criteria of application have been specified — or it combines words that are individually meaningful in a way that is counter-syntactical, producing no genuine statement.4 Bradley's Absolute, Heidegger's Being, the neo-vitalist's entelechy, the thing-in-itself: each falls under one or the other of these diagnoses.5
The verdict extends further than one might expect. It reaches not only speculative metaphysics — alleged knowledge by pure thinking or pure intuition that pretends to do without experience — but also any philosophy that starts from experience and then claims to infer something that transcends it.6 And it reaches normative theory as well: ethics and aesthetics, insofar as they purport to assert the objective validity of values, produce pseudo-statements for exactly the same reason.
Either empirical criteria are supplied for predicates like "good" and "beautiful," in which case the statement becomes a factual judgment rather than a value judgment, or no such criteria are supplied, in which case the statement is meaningless.7 What makes this elimination different from earlier anti-metaphysical movements — the nominalists, the empiricists of various stripes — is the precision the tools of modern logic make available.8
Earlier critics could say that metaphysics was sterile or extravagant; what logical analysis allows us to say is something sharper: that metaphysical utterances are meaningless in the strict sense, not merely pointless or false. A false statement is still meaningful; a pseudo-statement is not even that.9
The traditional disputes of philosophers are, for the most part, as unwarranted as they are unfruitful.Language, Truth and Logic, p. 32
Adoption of verifiability as a criterion for testing the significative statements of fact... For a statement of fact to be genuine some possible observations must be relevant to the determination of its truth or falsehood.Language, Truth and Logic, p. 26
Our thesis, now, is that logical analysis reveals the alleged statements of metaphysics to be pseudo-statements.Logical Positivism, p. 73
either they contain a word which is erroneously believed to have meaning, or the constituent words are meaningful, yet are put together in a counter-syntactical way, so that they do not yield a meaningful statement.Logical Positivism, p. 73
the neo-vitalist thesis of the directive presence of an 'entelechy' in organic processes... the question concerning the 'essence of causality'... the talk about the 'thing in itself'.Logical Positivism, pp. 88–89
This verdict hits, in the first place, any speculative metaphysics, any alleged knowledge by pure thinking or by pure intuition that pretends to be able to do without experience. But the verdict equally applies to the kind of metaphysics which, starting from experience, wants to acquire knowledge about that which transcends experience.Logical Positivism, pp. 88–89
Either empirical criteria are indicated for the use of 'good' and 'beautiful' and the rest of the predicates that are employed in the normative sciences, or they are not. In the first case, a statement containing such a predicate turns into a factual judgment, but not a value judgment; in the second case, it becomes a pseudo-statement.Logical Positivism, pp. 88–89
it is only now when the development of logic during recent decades provides us with a sufficiently sharp tool that the decisive step can be taken.Logical Positivism, p. 73
In the strict sense, however, a sequence of words is meaningless if it does not, within a specified language, constitute a statement... Such sentences are really meaningful, though they are pointless or false; for it is only meaningful sentences divisible into (theoretically) fruitful and sterile, true and false.Logical Positivism, p. 73