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.