Knowledge and The Problem of Knowledge, Ayer carries forward what is surely the finest inquiry of his century into these matters. They are matters that are not at all simple. Certainly he makes advances on the clarity of Hume and of John Stuart Mill and on the Vienna Circle of Logical Positivists. He also brings into better and quicker focus certain lines of thought in the work of his contemporary H. H. Price. If I may revert in passing to the subject of the man rather than his work the credit he gives to Price is the action of a honest man. Authors have managed to conceal debts to one another before now. The first chapter of The Foundations of Empirical Knowledge is a deft inquiry into the structure sides and aspects of the argument from illusion and above all into what conclusion or conclusions can be drawn from it. The role of sense-data is inquired into with an acute persistence that maintains a momentum which itself serves understanding. One conclusion in a bare sentence is that the description of our relation to the world in terms of sense-data is an arguable alternative to the description in terms of material things. There are alternative languages here. The second chapter opens the questions left nearly closed in what you have heard from me so far. These have to do with the exact nature of sense-data, and the exact nature of material things. The third chapter has centrally to do with the seeming privacy or subjectivity of sense-data, so different from the seeming public nature of physical things, and with what can be taken to follow from this.
Ayer's philosophy here is a phenomenalism which is to say a working-out of the idea that physical objects are somehow or other to be understood in terms of private and indubitable sense-data. This theory is his second and principal conclusion in the book. It is close to the idea that physical objects consist in sense-data in the sense that physical objects are entities constructed by us in thought out of sense-data. This work not his last on the subject was the object of the only sustained attack on Ayer's philosophy. This was by another distinguished Oxford professor J. L. Austin in the book Sense and Sensibilia (1962). It is an attack by a philosopher who exemplified both what was called ordinary language or linguistic philosophy and also a notable cleverness. If it caused hurt to Ayer it was surely conclusively replied to by him in the article Has Austin Refuted the Sense-Datum Theory? Another philosopher of rank to whom we will return in connection with the matter of Ayer's originality Anthony Quinton allows that Ayer shows that most of Austin's objections are captious. There is more in The Foundations of Empirical Knowledge than has been mentioned including a chapter having to do with causation and determinism in connection with perception. But let us pass on to what Ayer himself sometimes took to be his best book. It is The Problem of Knowledge, published in 1956 when his career as Grote Professor at University College London was near its zenith. What has been said already is nearly adequate as an introduction to it, but certainly it contains new thinking, theories that played no significant part in The Foundations of Empirical Knowledge. What is it to know something? That is no easy question. Ayer considers a number of conventional and tempting ideas, but comes to the succinct view that what it is for you to know something is for the proposition to be true, for you to believe it or be sure of it, and for you to have the right to believe it or be sure of it. It is a recommendation of this line of thought that it has been the point of departure for virtually all analyses of knowing since. It brings to mind that if it is mistaken there are mistakes that advance philosophy often because they put an idea or inclination clearly and forcefully. They are more important to progress in the subject than many contributions taken to have the recommendation of truth. Ayer's analysis of knowledge is a prelude to a new general view of the large subject-matter of The Foundations of Empirical Knowledge. This general view, which in fact covers more than that subject-matter, is located in a characterization of philosophical scepticism generally and of the possible responses to it. In a number of settings we seem to find ourselves with certain beginnings, premises reasons evidence or other beliefs according to the sceptic and also with larger beliefs so to speak that are related to them. We have what we take to be records and other evidence now in the present for the general conclusion that events took place in the past that there were past events, as against any events in particular. We have what we take to be excellent reasons, having to do with the speech and other behaviour of other humans around us, for the general conclusion that they have conscious lives like our own, that they too have ongoing minds. In these two cases and others, it seems, we make a transition from or go beyond the data, and can do so on the basis of only the data, there being no other grounds for the transition. The philosophical sceptic asks us for our justification. He points out that there is a gap. That is, the data do not logically entail the supposed conclusions it is not self-contradictory for me to accept as true all the propositions about your behaviours that in fact make me believe you are conscious but to doubt or deny that conclusion. Nor it is said is the conclusion produced by induction the method of science and much else. Such extrapolation or generalization from limited evidence of things going together cannot be got into operation with respect to the past and other minds. We never have particular experience of documents plus past or the behaviour of others plus their consciousness in order to get started towards a general conclusion. As you will anticipate the problem of reality the reality we take ourselves to know by way of perception or the problem of perception itself is of the same pattern, and perhaps is clarified by the pattern. We begin it seems, with sense-data, which are private and fleeting items, and we propose to end with material objects, public and lasting. Here too we face the challenge of the sceptic. And to proceed the responses to the gap or supposed gap are of the same sort as with the problems of the past and other minds. A first response with respect to the problem of reality is what Ayer refers to as naive realism, and others have spoken of more tolerantly or respectfully as direct realism. This view denies the initial proposition that we only have access to physical objects by way of our sense-data. Rather to say we experience pages in the most common understanding of the word is simply true. A second response is what Ayer here calls reductionism. It is in fact a somewhat altered phenomenalism. Here physical objects are again taken in a particular sense to consist of sense-data to be in a way constructions of or a matter of sense data. Talk of the two sorts of things is somehow equivalent. There is an effort to deny the importance of the gap between data and conclusion. While the realist asserts we can somehow make it to reality without the problematic stepping-stone of sense-data, the reductionist or phenomenalist in fact a very different philosophical character has it that there is no gap to cross. Both of sense-data and physical things are on the same side of what is no more than an invented or imagined division. These and other philosophies of external reality knowledge and perception are looked into in uniquely instructive ways in a long chapter in The Problem of Knowledge. The conclusion to which Ayer comes to state it again briskly is that in referring to physical objects as we do, we are advancing or engaging in a theory on the basis of the evidence of our senses a theory that has no source or ground other than private sense-data but does go beyond them. It is a particular kind of theoretical construction out of them. Distinguishing this more fully from the view of Foundations of Empirical Knowledge is not easily done, and cannot be done here. Again, if you are to come to have a grip on the issue lay hands on Foster's A. J. Ayer. As in the case of others of Ayer's books, The Problem of Knowledge has in it more than is needed for its main theme or line of argument. It looks at connected matters. In this case, there is a discussion of personal