Now, the reason does not follow the caprices of thought, it is subject to rules; it results from the properties of the images, those properties which we have above referred to, the material character of which we have recognised, and which are two in number--similarity and contiguity, as they are termed in the jargon of the schools. They are properties which have for their aim to bring things together, to unite, and to synthetise. They are unceasingly at work, and so apparent in their labour that they have long been known. We know, since the time of Aristotle, that two facts perceived at the same time reproduce themselves together in the memory--this is the law of contiguity; and that two facts perceived separately, but which are similar, are brought together in our mind--this is the law of similarity. Now, similarity and contiguity form by combination the essential part of all kinds of reasoning, and this reasoning, thus understood, works in a fashion which much resembles (we shall see exactly in what degree) a physical law. I wish to show this in a few words. What renders my demonstration difficult and perhaps obscure is, that we shall be obliged to bring together rather unexpectedly categories of phenomena which are generally considered separate. The distinctive attribute of the reason consists, as I have said, in the setting to work of these two elementary properties, similarity and contiguity. It consists, in fact, in extending continuity by similarity; in endowing with identical properties and similar accompaniments things which resemble each other; in other words, it consists in impliedly asserting that the moment two things are identical in one point they are so for all the rest. This will be fairly well understood by imagining what takes place when mental images having the above-mentioned properties meet. Suppose that B is associated with C, and that A resembles B. In consequence of their resemblance the passing from A to B is easy; and then B suggesting C by contiguity, it happens that this C is connected with A connected, though, in reality, they have never been tried together. I say they are associated on the basis of their relation to B, which is the rallying point. It is thus that, on seeing a piece of red-hot iron (A), I conclude it is hot (C), because I recollect distinctly or unconsciously another piece of red-hot iron (B), of which I once experienced the heat. It is this recollection B which logicians, in their analysis of logical, verbal, and formal argument, call the middle term. Our representation of the process of reasoning is not special to argument. It also expresses the process of invention, and every kind of progress from the known to the unknown. It is an activity which creates relations, which assembles and binds together, and the connections made between different representations are due to their partial identities, which act as solder to two pieces of metal.