AJ Ayer - Language, Truth and Logic - a summary

A-Level: Philosophy

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Kant said that human understanding became lost in contradiction when it ventured beyond experience. We ask how, if it is possible to know only what lies within sense-experience, it can be asserted that things lie beyond. As Wittgenstein says, "in order to draw a limit to thinking, we should have to think both sides of this limit", a truth which Bradley ingeniously twists in maintaining that anyone ready to prove metaphysics impossible is a brother metaphysician with a rival theory. What we accuse metaphysicians of is disobeying the rules governing the significant use of language.

We shall now proceed to formulate the criterion of verifiability which we use to test the genuineness of apparent statements of fact.

A sentence is factually significant if, and only if, we know how to verify the proposition it purports to express - that is, if we know what observations would lead us to accept the proposition as true or reject it as false. This procedure is central to the argument of this book.

A simple example would be the proposition that there are mountains on the other side of the moon. No rocket has yet enabled me to check this, but I know it to be decidable by observation. Therefore this proposition is verifiable in principle and is accordingly significant. On the other hand with such metaphysics as "the Absolute enters into, but is itself incapable of, evolution and progress" [FH Bradley] one cannot conceive of an observation which would determine whether the Absolute did or did not enter into evolution; the utterance has no literal significance.

A proposition is verifiable in the strong sense if, and only if, its truth could be conclusively established by experience. It is verified in the weak sense if it is possible for experience to render it probable.

If we adopt conclusive verifiability as our criterion of significance, our argument will prove too much, for even general laws such as "all men are mortal" or "arsenic is poisonous" cannot be established with certainty by any finite number of observations. Nor can we accept that a sentence should be allowed to be factually significant if, and only if, it expresses something definitely confutable by experience [Karl Popper]. A hypothesis cannot be conclusively confuted any more than it can be conclusively verified.

Accordingly, we fall back on the weaker sense of verification. We say that the question that must be asked about any putative statement of fact is not, Would any observations make its truth or falsehood certain? but simply, Would any observations be relevant to determination of its truth or falsehood? This criterion seems liberal enough. In contrast to conclusive verifiability it does not deny significance to general or historical propositions. Let us see what kinds of assertion it rules out.
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