Almost everyone, then, does believe that some perceptions other than his own, and which he himself does not directly perceive, are real; and believing this, he believes that something other than himself and what he directly perceives is real. But how do we know that anything exists except our own perceptions, and what we directly perceive? How do we know that there are any other people, who have perceptions in some respects similar to our own?
I believe that these two questions express very exactly the nature of the problem which it is my chief object, in this paper, to discuss. When I say these words to you, they will at once suggest to your minds the very question, to which I desire to find an answer; they will convey to you the very same meaning which I have before my mind, when I use the words. You will understand at once what question it is that I mean to ask. But, for all that, the words which I have used are highly ambiguous. If you begin to ask yourselves what I do mean by them, you will find that there are several quite different things which I might mean. And there is, I think, great danger of confusing these different meanings with one another. I think that philosophers, when they have asked this question in one sense, have often answered it in quite a different sense; and yet have supposed that the answer which they have given is an answer to the very same question which they originally asked. It is precisely because there is this ambiguity--this danger of confusion, in the words which I have used, that I have chosen to use them. I wish to point out as clearly as I can, not only what I do mean by them, but also some things which I do _not_ mean; and I wish to make it clear that the questions which I do _not_ mean to ask, are different questions from that which I do mean to ask.
I will take the second of my two questions, since there is in the other an additional ambiguity to which I do not now wish to call attention. My second question was: How do we know that there exist any other people who have perceptions in some respects similar to our own? What does this question mean?
Now I think you may have noticed that when you make a statement to another person, and he answers "How do you know that that is so?" he very often means to suggest that you do _not_ know it. And yet, though he means to suggest that you do not _know_ it, he may not for a moment wish to suggest that you do not _believe_ it, nor even that you have not that degree or kind of conviction, which goes beyond mere belief, and which may be taken to be essential to anything which can properly be called knowledge. He does not mean to suggest for a moment that you are saying something which you do not believe to be true, or even that you are not thoroughly convinced of its truth. What he does mean to suggest is that what you asserted was not _true_, even though you may not only have believed it but felt sure that it was true. He suggests that you don't _know_ it, in the sense that what you believe or feel sure of is not true.