We see, then, that Hume suggests, at least, three entirely different views as to the consequences of his original doctrine. His original doctrine was that, as regards matters of fact beyond the reach of our own actual observation, the knowledge of each of us is strictly limited to those for which we have a basis in our own experience. And his first view as to the consequences of this doctrine was that it does show us to be incapable of knowing a good many religious and philosophical propositions, which many men have claimed that they knew; but that it by no means denies our capacity of knowing the vast majority of facts beyond our own observation, which we all commonly suppose that we know. His second view, on the other hand, is that it cuts off at once all possibility of our knowing the vast majority of these facts; since he implies that we cannot have any basis in experience for asserting any external fact whatever—any fact, that is, except facts relating to our own actual past and future observations. And his third view is more sceptical still, since it suggests that we cannot really know any fact whatever, beyond the reach of our present observation or memory, even where we have a basis in experience for such a fact: it suggests that experience cannot ever let us know that any two things are causally connected, and therefore that it cannot give us knowledge of any fact based on this relation.
What are we to think of these three views, and of the original doctrine from which Hume seems to infer them?
As regards the last two views, it may perhaps be thought that they are too absurd to deserve any serious consideration. It is, in fact, absurd to suggest that I do not know any external facts whatever; that I do not know, for instance, even that there are any men beside myself. And Hume himself, it might seem, does not seriously expect or wish us to accept these views. He points out, with regard to all such excessively sceptical opinions that we cannot continue to believe them for long together—that, at least, we cannot, for long together, avoid believing things flatly inconsistent with them. The philosopher may believe, when he is philosophising, that no man knows of the existence of any other man or of any material object; but at other times he will inevitably believe, as we all do, that he does know of the existence of this man and of that, and even of this and that material object. There can, therefore, be no question of making all our beliefs consistent with such views as this of never believing anything that is inconsistent with them. And it may, therefore, seem useless to discuss them. But in fact, it by no means follows that, because we are not able to adhere consistently to a given view, therefore that view is false; nor does it follow that we may not sincerely believe it, whenever we are philosophising, even though the moment we cease to philosophise, or even before, we may be forced to contradict it. And philosophers do, in fact, sincerely believe such things as this—things which flatly contradict the vast majority of the things which they believe at other times. Even Hume, I think, does sincerely wish to persuade us that we cannot know of the existence of external material objects—that this is a philosophic truth, which we ought, if we can, so long as we are philosophising, to believe. Many people, I think, are certainly tempted, in their philosophic moments, to believe such things; and, since this is so, it is, I think, worth while to consider seriously what arguments can be brought against such views. It is worth while to consider whether they are views which we ought to hold as philosophical opinions, even if it be quite certain that we shall never be able to make the views which we entertain at other times consistent with them. And it is the more worth while, because the question how we can prove or disprove such extreme views as these, has a bearing on the question how we can, in any case whatever, prove or disprove that we do really know, what we suppose ourselves to know.
What arguments, then, are there for or against the extreme view that no man can know any external fact whatever; and the still more extreme view that no man can know any matter of fact whatever, except those which he is directly observing at the moment, or has observed in the past and now remembers?
It may be pointed out, in the first place, that, if these views are true, then at least no man can possibly know them to be so. What these views assert is that I cannot know any external fact whatever. It follows, therefore, that I cannot know that there are any other men, beside myself, and that they are like me in this respect. Any philosopher who asserts positively that other men, equally with himself, are incapable of knowing any external facts, is, in that very assertion, contradicting himself, since he implies that he does know a great many facts about the knowledge of other men. No one, therefore, can be entitled to assert positively that human knowledge is limited in this way, since, in asserting it positively, he is implying that his own knowledge is not so limited. It cannot be proper, even in our philosophic moments, to take up such an attitude as this.
No one, therefore, can know positively that men in general, are incapable of knowing external facts. But still, although we cannot know it, it remains possible that the view should be a true one. Nay, more, it remains possible that a man should know that he himself is incapable of knowing any external facts, and that, if there are any other men whose faculties are only similar to his own, they also must be incapable of knowing any. The argument just used obviously does not apply against such a position as this. It only applies against the position that men in general positively are incapable of knowing external facts: it does not apply against the position that the philosopher himself is incapable of knowing any, or against the position that there are possibly other men in the same case, and that, if their faculties are similar to the philosopher's, they certainly would be in it. I do not contradict myself by maintaining positively that I know no external facts, though I do contradict myself if I maintain that I am only one among other men, and that no man knows any external facts. So far, then, as Hume merely maintains that he is incapable of knowing any external facts, and that there may be other men like him in this respect, the argument just used is not valid against his position. Can any conclusive arguments be found against it?
It seems to me that such a position must, in a certain sense, be quite incapable of disproof. So much must be granted to any sceptic who feels inclined to hold it. Any valid argument which can be brought against it must be of the nature of a petitio principii: it must beg the question at issue. How is the sceptic to prove to himself that he does know any external facts? He can only do it by bringing forward some instance of an external fact, which he does know; and, in assuming that he does know this one, he is, of course, begging the question. It is therefore quite impossible for any one to prove, in one strict sense of the term, that he does know any external facts. I can only prove that I do, by assuming that in some particular instance, I actually do know one. That is to say, the so-called proof must assume the very thing which it pretends to prove. The only proof that we do know external facts lies in the simple fact that we do know them. And the sceptic can, with perfect internal consistency, deny that he does know any. But it can, I think, be shown that he has no reason for denying it. And in particular it may, I think, be easily seen that the arguments which Hume uses in favour of this position have no conclusive force.
To begin with, his arguments, in both cases, depend upon the two original assumptions, (1) that we cannot know any fact, which we have not observed, unless we know it to be causally connected with some fact which we have observed, and (2) that we have no reason for assuming any causal connection, except where we have experienced some instances of conjunction between the two facts connected. And both of these assumptions may, of course, be denied. It is just as easy to deny them, as to deny that I do know any external facts. And, if these two assumptions did really lead to the conclusion that I cannot know any, it would, I think, be proper to deny them: we might fairly regard the fact that they led to this absurd conclusion as disproving them. But, in fact, I think it may be easily seen that they do not lead to it.
Let us consider, first of all, Hume's most sceptical argument (the argument which he merely suggests). This argument suggests that, since our only reason for supposing two facts to be causally connected is that we have found them constantly conjoined in the past, and since it does not follow from the fact that they have been conjoined ever so many times, that they always will be so, therefore we cannot know that they always will be so, and hence cannot know that they are causally connected. But obviously the conclusion does not follow. We must, I think, grant the premiss that, from the fact that two things have been conjoined, no matter how often, it does not strictly follow that they always are conjoined. But it by no means follows from this that we may not know that, as a matter of fact, when two things are conjoined sufficiently often, they are also always conjoined. We may quite well know many things which do not logically follow from anything else which we know. And so, in this case, we may know that two things are causally connected, although this does not logically follow from our past experience, nor yet from anything else that we know. And, as for the contention that our belief in causal connections is merely based on custom, we may, indeed, admit that custom would not be a sufficient reason for concluding the belief to be true. But the mere fact (if it be a fact) that the belief is only caused by custom, is also no sufficient reason for concluding that we can not know it to be true. Custom may produce beliefs, which we do know to be true, even though it be admitted that it does not necessarily produce them.
And as for Hume's argument to prove that we can never know any external object to be causally connected with anything which we actually observe, it is, I think, obviously fallacious. In order to prove this, he has, as he recognises, to disprove both of two theories. He has, first of all, to disprove what he calls the vulgar theory—the theory that we can know the very things which we see or feel to be external objects; that is to say, can know that these very things exist at times when we do not observe them. And even here, I think, his arguments are obviously inconclusive. But we need not stay to consider them, because, in order to prove that we cannot know any external objects, he has also to disprove what he calls the philosophic theory—the theory that we can know things which we do observe, to be caused by external objects which we never observe. If, therefore, his attempt to disprove this theory fails, his proof that we cannot know any external objects also fails; and I think it is easy to see that his disproof does fail. It amounts merely to this: That we cannot, ex hypothesi, ever observe these supposed external objects, and therefore cannot observe them to be constantly conjoined with any objects which we do observe. But what follows from this? His own theory about the knowledge of causal connection is not that in order to know A to be the cause of B, we must have observed A itself to be conjoined with B; but only that we must have observed objects like A to be constantly conjoined with objects like B. And what is to prevent an external object from being like some object which we have formerly observed? Suppose I have frequently observed a fact like A to be conjoined with a fact like B: and suppose I now observe B, on an occasion when I do not observe anything like A. There is no reason, on Hume's principles, why I should not conclude that A does exist on this occasion, even though I do not observe it; and that it is, therefore, an external object. It will, of course, differ from any object which I have ever observed, in respect of the simple fact that it is not observed by me, whereas they were. There is, therefore, this one respect in which it must be unlike anything which I have ever observed. But Hume has never said anything to show that unlikeness in this single respect is sufficient to invalidate the inference. It may quite well be like objects which I have observed in all other respects; and this degree of likeness may, according to his principles, be quite sufficient to justify us in concluding its existence. In short, when Hume argues that we cannot possibly learn by experience of the existence of any external objects, he is, I think, plainly committing the fallacy of supposing that, because we cannot, ex hypothesis have ever observed any object which actually is "external," therefore we can never have observed any object like an external one. But plainly we may have observed objects like them in all respects except the single one that these have been observed whereas the others have not. And even a less degree of likeness than this would, according to his principles, be quite sufficient to justify an inference of causal connection.