Ayer’s Language, Truth and Logic
A.J. Ayer Language Truth and Logic Victor Gallancz 1936
I have a 2001 Penguin Books edition, which includes a 1946 appendix. According to the back cover:
Ayer argued that factual propositions were only meaningful if they could be verified by direct experience. This enabled him to reject metaphysical claims … as, literally, nonsense. [This book] became a classic text, which not only revitalized British Philosophy but which set a new standard for philosophical debate.
Truth and Probability
It is only tautologies that are certain.
[We] test the validity of an empirical hypothesis by seeing whether it actually fulfils the function which it is designed to fulfil. [The] function of an empirical hypothesis is to enable us to anticipate the future.
[To] be rational is simply to employ a self-consistent accredited procedure in the formation of all one’s beliefs. The fact that the procedure, by reference to which we now determine whether a belief is rational, may subsequently forfeit our confidence, does not in any way detract from the rationality of adopting it now. For we define a rational belief as one which is arrived at by the methods which we now consider reliable.
To say that an observation increases the probability of a hypothesis … is equivalent to saying that the observation increases the degree of confidence with which it is rational to entertain the hypothesis [where] the rationality of a belief is defined, not by reference to any absolute standard, but by reference to part of our own actual practice.
[A] statement is directly verifiable if it is either itself an observation-statement, or is such that in conjunction with certain other premises it entails one or more observation statements it entails at least one observation-statement which is not deducible from these other premises alone. …
[The] principles of verification [requires] that of a literally meaningful statement, that is not analytic, that it should be directly or indirectly verifiable … .
I am tempted to summarise Ayer’s view as being that one ‘should’ form beliefs and then act upon them, and as being consistent with Bayes’ rule. This often seem to me common sense. But it seems that we may not be entirely confident in ‘the procedure, by reference to which we now determine whether a belief is rational’. Alternatively, if we recognize that such a procedure ‘may subsequently forfeit our confidence’, how is it that we are entitled to consider it reliable? Thus in the initial book Ayer presents us with a conundrum. The solution, in the appendix, is that we regard a statement as verified if it is useful in the sense of correctly predicting something that we couldn’t otherwise predict. I think this does capture something of how many people do think. But is it wise?
I tend to agree with Keynes’ Treatise on Probability, that some caveat is needed. For example, if the procedure for establishing rationality has a good track record. It may be significant if the procedure is ‘generally accepted, orthodox’, but this hardly seems decisive. In economics, for example, it seems to me that there is a poor track record, and no grounds for supposing that what is ‘generally accepted, orthodox’ is going to be reliable. But I do think the notion of probability is a useful one. We do not suppose that a probability estimate is necessarily reliable, only that it reflects what we currently consider to be best practice. It is a separate question as to whether the best current practice is appropriate to the case at hand. Thus it is quite possible that those who were predicting growth of about 2% through 2008 were fully justified in Ayers sense, and knowing now that there was a financial crash is no grounds for criticism. What – with hindsight – ought to have been questioned was the basis for those expectations. Were there empirical grounds for believing them to be adequate? In Ayer’s terms this is a question about how the ‘pseudo-mathematical’ procedures came to be accredited. My view is that it is not enough to use a procedure unless and until it ‘forfeits our confidence’: we should be actively critiquing the procedure, so that we reject or amend it as early as is practicable.