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?