There never comes a point where a theory can be said to be true. The most that one can claim for any theory is that it has shared the successes of all its rivals and that it has passed at least one test which they have failed.
Interpretation
The quote suggests that no theory can be definitively proven true; rather, their validity is based on comparative success and ability to withstand challenges.
A.J. Ayer's quote emphasizes the idea that scientific theories are never conclusively true but are instead measured by their effectiveness against competing theories. A theory's credibility is not a permanent state but a reflection of its ability to succeed in tests where others have failed, highlighting the provisional nature of knowledge in the philosophy of science.
In practice
In a scientific discussion about the nature of theories, this quote illustrates the temporary nature of knowledge.
If 'god' is a metaphysical term, then it cannot be even probable that a god exists. For to say that 'God exists' is to make a metaphysical utterance which cannot be either true or false. And by the same criterion, no sentence which purports to describe the nature of a transcendent god can possess any literal significance.
In other words, the propositions of philosophy are not factual, but linguistic in character - that is, they do not describe the behaviour of physical, or even mental, objects; they express definitions, or the formal consequences of definitions. Accordingly we may say that philosophy is a department of logic. For we will see that the characteristic mark of a purely logical enquiry, is that it is concerned with the formal consequences of our definitions and not with questions of empirical fact.
It is time, therefore, to abandon the superstition that natural science cannot be regarded as logically respectable until philosophers have solved the problem of induction. The problem of induction is, roughly speaking, the problem of finding a way to prove that certain empirical generalizations which are derived from past experience will hold good also in the future.
The ground for taking ignorance to be restrictive of freedom is that it causes people to make choices which they would not have made if they had seen what the realization of their choices involved.
The only possible basis for a sound morality is mutual tolerance and respect: tolerance of one another’s customs and opinions; respect for one another’s rights and feelings; awareness of one another’s needs.
The traditional disputes of philosophers are, for the most part, as unwarranted as they are unfruitful.
To demand that a person pee in a cup whenever you wish him to, without a documented reason to suspect that he has been using an illegal drug, is intolerable in our republic. You are saying to him, "I wonder if you are not behaving in a way that I approve of. Convince me that you indeed are. Outrageous. Intolerable.
As man advances in civilization, and small tribes are united into larger communities, the simplest reason would tell each individual that he ought to extend his social instincts and sympathies to all members of the same nation, though personally unknown to him. This point being once reached, there is only an artificial barrier to prevent his sympathies extending to the men of all nations and races.
When we adopt—and when we encourage a culture of adoption in our churches and communities—we’re picturing something that’s true about our God. We, like Jesus, see what our Father is doing and do likewise (John 5:19). And what our Father is doing, it turns out, is fighting for orphans, making them sons and daughters.
To look for a single general theory of how to decide the right thing to do is like looking for a single theory of how to decide what to believe.
There is a Passion natural to the Mind of man, especially a free Man, which renders him impatient of Restraint.
Many things, for aught I know, may exist, whereof neither I nor any other man hath or can have any idea or notion whatsoever.
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