But can such a definition as the above really stand? Does it represent at all the sort of thing that would commonly be called knowledge? I think it does in the main, but there is need of some discussion to make this clear. In some cases, the definition is obviously applicable. These are the cases that are analogous to the rat in the maze, the consideration of which led us to our definition. Do you “know” the way from Trafalgar Square to St. Pancras? Yes, if you can walk it without taking any wrong turnings. In practice, you can give verbal proof of such knowledge, without actually having to walk the distance; but that depends upon the correlation of names with streets, and is part of the process of substituting words for things. There may, it is true, come doubtful cases. I was once on a bus in Whitehall, and my neighbour asked “What street is this?” I answered him, not without surprise at his ignorance. He then said, “What building is that?” and I replied “The Foreign Office”. To this he retorted, “but I thought the Foreign Office was in Downing Street”. This time, it was his knowledge that surprised me. Should we say that he knew where the Foreign Office is? The answer is yes or no according to his purpose. From the point of view of sending a letter to it, he knew; from the point of view of walking to it, he did not know. He had, in fact, been a British Consul in South America, and was in London for the first time. But now let us come to cases less obviously within the scope of our definition. The reader “knows” that Columbus crossed the ocean in 1492. What do we mean by saying that he “knows” this? We mean, no doubt, primarily that writing down this statement is the way to pass examinations, which is just as useful to us as getting out of the maze is to the rat. But we do not mean only this. There is historical evidence of the fact, at least I suppose there is. The historical evidence consists of printed books and manuscripts. Certain rules have been developed by historians as to the conditions in which statements in books or manuscripts may be accepted as true, and the evidence in our case is (I presume) in accordance with these rules. Historical facts often have importance in the present; for example, wills, or laws not yet repealed. The rules for weighing historical evidence are such as will, in general bring out self-consistent results. Two results are self-consistent when, in relation to a desire to which both are relevant, they enjoin the same action, or actions which can form part of the one movement towards the goal. At Coton, near Cambridge, there is (or was in my time) a signpost with two arms pointing in diametrically opposite directions, and each arm said “To Cambridge”. This was a perfect example of self-contradiction, since the two arms made statements representing exactly opposite actions. And this case illustrates why self-contradiction is to be avoided. But the avoidance