Hume does not, therefore, bring forward any arguments at all sufficient to prove either that he cannot know any one object to be causally connected with any other or that he cannot know any external fact. And, indeed, I think it is plain that no conclusive argument could possibly be advanced in favour of these positions. It would always be at least as easy to deny the argument as to deny that we do know external facts. We may, therefore, each one of us, safely conclude that we do know external facts; and, if we do, then there is no reason why we should not also know that other men do the same. There is no reason why we should not, in this respect, make our philosophical opinions agree with what we necessarily believe at other times. There is no reason why I should not confidently assert that I do really know some external facts, although I cannot prove the assertion except by simply assuming that I do. I am, in fact, as certain of this as of anything; and as reasonably certain of it. But just as I am certain that I do know some external facts, so I am also certain that there are others which I do not know. And the question remains: Does the line between the two fall, where Hume says it falls? Is it true that the only external facts I know are facts for which I have a basis in my own experience? And that I cannot know any facts whatever, beyond the reach of my own observation and memory, except those for which I have such a basis?
This, it seems to me, is the most serious question which Hume raises. And it should be observed that his own attitude towards it is very different from his attitude towards the sceptical views which we have just been considering. These sceptical views he did not expect or wish us to accept, except in philosophic moments. He declares that we cannot, in ordinary life, avoid believing things which are inconsistent with them; and, in so declaring, he, of course, implies incidentally that they are false: since he implies that he himself has a great deal of knowledge as to what we can and cannot believe in ordinary life. But, as regards the view that our knowledge of matters of fact beyond our own observation is entirely confined to such as are founded on experience, he never suggests that it is impossible that all our beliefs should be consistent with this view, and he does seem to think it eminently desirable that they should be. He declares that any assertion with regard to such matters, which is not founded on experience, can be nothing but "sophistry and illusion"; and that all books which are composed of such assertions should be "committed to the flames." He seems, therefore, to think that here we really have a test by which we may determine what we should or should not believe, on all occasions: any view on such matters, for which we have no foundation in experience, is a view which we cannot know to be even probably true, and which we should never accept, if we can help it. Is there any justification for this strong view?
It is, of course, abstractly possible that we do really know, without the help of experience, some matters of fact, which we never have observed. Just as we know matters of fact, which we have observed, without the need of any further evidence, and just as we know, for instance, that 2+2=4, without the need of any proof, it is possible that we may know, directly and immediately, without the need of any basis in experience, some facts which we never have observed. This is certainly possible, in the same sense in which it is possible that I do not really know any external facts: no conclusive disproof can be brought against either position. We must make assumptions as to what facts we do know and do not know, before we can proceed to discuss whether or not all of the former are based on experience; and none of these assumptions can, in the last resort, be conclusively proved. We may offer one of them in proof of another; but it will always be possible to dispute the one which we offer in proof. But there are, in fact, certain kinds of things which we universally assume that we do know or do not know, just as we assume that we do know some external facts; and if among all the things which we know as certainly as this, there should turn out to be none for which we have no basis in experience, Hume's view would I think, be as fully proved as it is capable of being. The question is: Can it be proved in this sense? Among all the facts beyond our own observation, which we know most certainly, are there any which are certainly not based upon experience? For my part, I confess, I cannot feel certain what is the right answer to this question: I cannot tell whether Hume was right or wrong. But if he was wrong—if there are any matters of fact, beyond our own observation, which we know for certain, and which yet we know directly and immediately, without any basis in experience, we are, I think, faced with an eminently interesting problem. For it is, I think, as certain as anything can be that there are some kinds of facts with regard to which Hume was right—that there are some kinds of facts which we cannot know without the evidence of experience. I could not know, for instance, without some such evidence, such a fact as that Julius Caesar was murdered. For such a fact I must, in the first instance, have the evidence of other persons; and if I am to know that their evidence is trustworthy, I must have some ground in experience for supposing it to be so. There are, therefore, some kinds of facts which we cannot know without the evidence of experience and observation. And if it is to be maintained that there are others, which we can know without any such evidence, it ought to be pointed out exactly what kind of facts these are, and in what respects they differ from those which we cannot know without the help of experience. Hume gives us a very clear division of the kinds of propositions which we can know to be true. There are, first of all, some propositions which assert "relations of ideas "; there are, secondly, propositions which assert "matters of fact" which we ourselves are actually observing, or have observed and now remember; and there are, thirdly, propositions which assert "matters of fact" which we have never actually observed, but for believing in which we have some foundation in our past observations. And it is, I think, certain that some propositions, which we know as certainly as we know anything, do belong to each of these three classes. I know, for instance, that twice two are four; I know by direct observation that I am now seeing these words, that I am writing, and by memory that this afternoon I saw St. Paul's; and I know also that Julius Caesar was murdered, and I have some foundation in experience for this belief, though I did not myself witness the murder. Do any of those propositions, which we know as certainly as we know these and their like, not belong to either of these three classes? Must we add a fourth class consisting of propositions which resemble the two last, in respect of the fact that they do assert "matters of fact," but which differ from them, in that we know them neither by direct observation nor by memory, nor yet as a result of previous observations? There may, perhaps, be such a fourth class; but, if there is, it is, I think, eminently desirable that it should be pointed out exactly what propositions they are which we do know in this way; and this, so far as I know, has never yet been done, at all clearly, by any philosopher.
[THE STATUS OF SENSE-DATA]
The term "sense-data" is ambiguous; and therefore I think I had better begin by trying to explain what the class of entities is whose status I propose to discuss.
There are several different classes of mental events, all of which, owing to their intrinsic resemblance to one another in certain respects, may, in a wide sense, be called "sensory experiences," although only some among them would usually be called "sensations." There are (1) those events, happening in our minds while we are awake, which consist in the experiencing of one of those entities, which are usually called "images," in the narrowest sense of the term. Everybody distinguishes these events from sensations proper; and yet everybody admits that "images" intrinsically resemble the entities which are experienced in sensations proper in some very important respect. There are (2) the sensory experiences we have in dreams, some of which would certainly be said to be experiences of images, while others might be said to be sensations. There are (3) hallucinations, and certain classes of illusory sensory experiences. There are (4) those experiences, which used to be called the having of "after-images," but which psychologists now say ought rather to be called "after-sensations." And there are, finally, (5) that class of sensory experiences, which are immensely commoner than any of the above, and which may be called sensations proper, if we agree to use this term in such a way as to exclude experiences of my first four sorts.