[From A. J. Ayer, The Problem of Knowledge (Harmondsworth: Penguin, 1956). Sir Alfred Jules (“Freddie”) Ayer (1910–1989) was an eminent English philosopher.]
	A. J. AYER Knowing as Having the Right to Be Sure
1	The answers which we have found for the questions we have so far been discussing have not yet put us in a position to give a complete account of what it is to know that something is the case. The first requirement is that what is known should be true, but this is not sufficient; not even if we add to it the further condition that one must be completely sure of what one knows. For it is possible to be completely sure of something which is in fact true, but yet not to know it. The circumstances may be such that one is not entitled to be sure. For instance, a superstitious person who had inadvertently walked under a ladder might be convinced as a result that he was about to suffer some misfortune; and he might in fact be right. But it would not be correct to say that he knew that this was going to be so. He arrived at his belief by a process of reasoning which would not be generally reliable; so, although his prediction came true, it was not a case of knowledge. Again, if someone were fully persuaded of a mathematical proposition by a proof which could be shown to be invalid, he would not, without further evidence, be said to know the proposition, even though it was true. But while it is not hard to find examples of true and fully confident beliefs which in some ways fail to meet the standards required for knowledge, it is not at all easy to determine exactly what these standards are.
2	One way of trying to discover them would be to consider what would count as satisfactory answers to the question How do you know? Thus people may be credited with knowing truths of mathematics or logic if they are able to give a valid proof of them, or even if, without themselves being able to set out such a proof, they have obtained this information from someone who can. Claims to know empirical statements may be upheld by a reference to perception, or to memory, or to testimony, or to historical records, or to scientific laws. But such backing is not always strong enough for knowledge. Whether it is so or not depends upon the circumstances of the particular case. If I were asked how I knew that a physical object of a certain sort was in such and such a place, it would, in general, be a sufficient answer for me to say that I could see it; but if my eyesight were bad and the light were dim, this answer might not be sufficient. Even though I was right, it might still be said that I did not really know that the object was there. If I have a poor memory and the event which I claim to remember is remote, my memory of it may still not amount to knowledge, even though in this instance it does not fail me. If a witness is unreliable, his unsupported evidence may not enable us to know that what he says is true, even in a case where we completely trust him and he is not in fact deceiving us. In a given instance it is possible to decide whether the backing is strong enough to justify a claim to knowledge. But to say in general how strong it has to be would require our drawing up a list of the conditions under which perception, or memory, or testimony, or other forms of evidence are reliable. And this would be a very complicated matter, if indeed it could be done at all.
3	Moreover, we cannot assume that, even in particular instances, an answer to the question How do you know? will always be forthcoming. There may very well be cases in which one knows that something is so without its being possible to say how one knows it. I am not so much thinking now of claims to know facts of immediate experience, statements like ‘I know that I feel pain’, which raise problems of their own into which we shall enter later on. In cases of this sort it may be argued that the question how one knows does not arise. But even when it clearly does arise, it may not find an answer. Suppose that someone were consistently successful in predicting events of a certain kind, events, let us say, which are not ordinarily thought to be predictable, like the results of a lottery. If his run of successes were sufficiently impressive, we might very well come to say that he knew which number would win, even though he did not reach this conclusion by any rational method, or indeed by any method at all. We might say that he knew it by intuition, but this would be to assert no more than that he did know it but that we could not say how. In the same way, if someone were consistently successful in reading the minds of others without having any of the usual sort of evidence, we might say that he knew these things telepathically. But in default of any further explanation this would come down to saying merely that he did know them, but not by an ordinary means. Words like ‘intuition’ and ‘telepathy’ are brought in just to disguise the fact that no explanation has been found.
4	But if we allow this sort of knowledge to be even theoretically possible, what becomes of the distinction between knowledge and true belief? How does our man who knows what the results of the lottery will be differ from one who only makes a series of lucky guesses? The answer is that, so far as the man himself is concerned, there need not be any difference. His procedure and his state of mind, when he is said to know what will happen, may be exactly the same as when it is said that he is only guessing. The difference is that to say that he knows is to concede to him the right to be sure, while to say that he is only guessing is to withhold it. Whether we make this concession will depend upon the view which we take of his performance. Normally we do not say that people know things unless they have followed one of the accredited routes to knowledge. If someone reaches a true conclusion without appearing to have any adequate basis for it, we are likely to say that he does not really know it. But if he were repeatedly successful in a given domain, we might very well come to say that he knew the facts in question, even though we could not explain how he knew them. We should grant him the right to be sure, simply on the basis of his success. This is, indeed, a point on which people’s views might be expected to differ. Not everyone would regard a successful run of predictions, however long sustained, as being by itself a sufficient backing for a claim to knowledge. And here there can be no question of proving that this attitude is mistaken. Where there are recognized criteria for deciding when one has the right to be sure, anyone who insists that their being satisfied is still not enough for knowledge may be accused, for what the charge is worth, of misusing the verb ‘to know’. But it is possible to find, or at any rate to devise, examples which are not covered in this respect by any established rule of usage. Whether they are to count as instances of knowledge is then a question which we are left free to decide.
5	It does not, however, matter very greatly which decision we take. The main problem is to state and assess the grounds on which these claims to knowledge are made, to settle, as it were, the candidate’s marks. It is a relatively unimportant question what titles we then bestow upon them. So long as we agree about the marking, it is of no great consequence where we draw the line between pass and failure, or between the different levels of distinction. If we choose to set a very high standard, we may find ourselves committed to saying that some of what ordinarily passes for knowledge ought rather to be described as probable opinion. And some critics will then take us to task for flouting ordinary usage. But the question is purely one of terminology. It is to be decided, if at all, on grounds of practical convenience. [. . .]
7	I conclude then that the necessary and sufficient conditions for knowing that something is the case are first that what one is said to know be true, secondly that one be sure of it, and thirdly that one should have the right to be sure. This right may be earned in various ways; but even if one could give a complete description of them it would be a mistake to try to build it into the definition of knowledge, just as it would be a mistake to try to incorporate our actual standards of goodness into a definition of good. And this being so, it turns out that the questions which philosophers raise about the possibility of knowledge are not all to be settled by discovering what knowledge is. For many of them reappear as questions about the legitimacy of the title to be sure. They need to be severally examined; and this is the main concern of what is called the theory of knowledge.