It is common to speak of images as “centrally excited”, as opposed to sensations, which are excited by a stimulus to some sense organ. In essence this is quite correct, but there is need of some caution in interpreting the phrase. Sensations also have _proximate_ causes in the brain; images also may be due to some excitement of a sense-organ, when they are roused by a sensation through association. But in such cases there is nothing to explain their occurrence except the past experience and its effect on the brain. They will not be aroused by the same stimulus in a person with similar sense-organs but different past experience. The connection with past experience is clearly known; it is, however, an explanatory hypothesis, not directly verifiable in the present state of knowledge, to suppose that this connection works through an effect of the past experience on the brain. This hypothesis must be regarded as doubtful, but it will save circumlocution to adopt it. I shall therefore not repeat, on each occasion, that we cannot feel sure it is true. In general, where the causal connection with past experience is obvious, we call an occurrence “mnemic”, without implying this or that hypothesis as to the explanation of mnemic phenomena. It is perhaps worth while to ask how we know that images are like the sensations which are their prototypes. The difficulty of this question arises as follows. Suppose you call up an image of the Brooklyn Bridge, and you are convinced that it is like what you see when you look at Brooklyn Bridge. It would seem natural to say that you know the likeness because you remember Brooklyn Bridge. But remembering is often held to involve, as an essential element, the occurrence of an image which is regarded as referring to a prototype. Unless you can remember without images, it is difficult to see how you can be sure that images resemble prototypes. I think that in fact you cannot be sure, unless you can find some indirect means of comparison. You might, for example, have photographs of Brooklyn Bridge taken from a given place on two different days, and find them indistinguishable, showing that Brooklyn Bridge has not changed in the interval. You might see Brooklyn Bridge on the first of these days, remember it on the second, and immediately afterwards look at it. In looking at it, you might find every detail coming to you with a feeling of expectedness, or you might find some details coming with a feeling of surprise. In this case you would say that your image had been wrong as regards the details which were surprising. Or, again, you might make a picture of Brooklyn Bridge on paper, from memory, and then compare it with the original or a photograph. Or you might content yourself by writing down a description of it in words, and verifying its accuracy by direct observation. Innumerable methods of this kind can be devised by which you can test the likeness of an image to its prototype. The result is that there is