The word “knowledge” is very ambiguous. We say that Watson’s rats “know” how to get out of mazes, that a child of three “knows” how to talk, that a man “knows” the people with whom he is acquainted, that he “knows” what he had for breakfast this morning, and that he “knows” when Columbus first crossed the ocean. French and German are less ambiguous, since each has two words for different kinds of “knowing”, which we tend to confuse in our thoughts because we confuse them in our language. I shall not attempt as yet to deal with knowledge in general, but rather with certain less general concepts which would ordinarily be included under “knowledge”. And first of all I will deal with perception--not as it appears to the perceiver, but as it can be tested by an outside observer. Let us try, first, to get a rough preliminary view of the sort of thing we are going to mean by “perception”. One may say that a man “perceives” anything that he notices through his senses. This is not a question of the sense-organs alone, though they are a necessary condition. No man can perceive by sight what is not in his field of vision, but he may look straight at a thing without perceiving it. I have frequently had the experience--supposed to be characteristic of philosophers--of looking everywhere for my spectacles although they were before my eyes when my search began. We cannot therefore tell what a man is perceiving by observing his sense-organs alone, though they may enable us to know that he is _not_ perceiving something. The observer can only know that a man is perceiving something if the man reacts in some appropriate manner. If I say to a man “please pass the mustard” and he thereupon passes it, it is highly probable that he perceived what I said, although it _may_ of course be a mere coincidence that he passed it at that moment. But if I say to him “the telephone number you want is 2467” and he proceeds to call that number, the odds against his doing so by mere chance are very great--roughly 10,000 to 1. And if a man reads aloud out of a book, and I look over his shoulder and perceive the same words, it becomes quite fantastic to suppose that he does not perceive the words he is uttering. We can thus in many cases achieve practical certainty as to some of the things that other people are perceiving.