understand what they hear in it; but this is perhaps not really a case in point. The difficulty we are considering arises from the fact that a human body, unlike a scientific instrument, is perpetually changing its reaction to a given stimulus, under the influence of the law of association. Moreover, the human body is always doing something. How, then, are we to know whether what it is doing is the result of a given stimulus or not? In most cases, however, this difficulty is not very serious, particularly when we are dealing with people old enough to speak. When you go to the oculist he asks you to read a number of letters growing gradually smaller; at some point you fail. Where you have succeeded, he knows that you have perceived enough to make out what letter it is. Or you take a pair of compasses and press the points into a man’s back, asking him if he feels two pricks or only one. He may say one when the two points are near together; if he is on his guard against this error he may say two when in fact there is only one. But if the points are sufficiently far apart he will never make a mistake. That is to say, the bodily movement consisting in pronouncing the word “two” will invariably result from a certain stimulus. (Invariably, I mean, for a given subject on a given day.) This entitles us to say that the man can perceive that there are two points provided they are not too near together. Or you say: “What can you see on the horizon?” One man says, “I see a ship”. Another says, “I see a steamer with two funnels”. A third says, “I see a Cunarder going from Southampton to New York”. How much of what these three people say is to count as perception? They may all three be perfectly right in what they say, and yet we should not concede that a man can “perceive” that the ship is going from Southampton to New York. This, we should say, is inference. But it is by no means easy to draw a line; some things which are, in an important sense, inferential, must be admitted to be perceptions. The man who says “I see a ship” is using inference. Apart from experience, he only sees a queerly shaped dark dot on a blue background. Experience has taught him that that sort of dot “means” a ship; that is to say, he has a conditioned reflex which causes him to utter, aloud or to himself, the word “ship” when his eye is stimulated in a certain way. To disentangle what is due to experience, and what not, in the perceptions of an adult, is a hopeless task. Practically, if a word comes without previous verbal intermediaries, the ordinary man would include what the word means in the perception, while he would not do so if the man arrives at the word after verbal preliminaries, overt or internal. But this is itself a question of familiarity. Show a child a pentagon, and he will have to count the sides to know how many there are; but after a little experience of geometrical figures, the word “pentagon” will arise without any previous words. And in any