the association of unlikes involves the utmost degree of resistance. It follows that where unlikes are associated, the resistance offered to their union will tend (1), where they are forcibly held in association, to make them likes; and (2) where they are free to move, to dissociate them. This law of assimilation, as we may briefly call it, finds illustration over so vast a field that it may fairly be described as a universal character.
If we begin, then, with the most complex of all the phenomena known to us, our first illustrations will be drawn from the realm of mind. The fact that cognition is a process of the association of like and the dissociation of unlike impressions, and the further fact that all the activities of thought, from reasoning of the lowest to reasoning of the highest kind, involve the association of like and dissociation of unlike elements—these are psychological truths of the utmost certainty. At the outset of all knowing is the indispensable condition that unless we can connect the thing perceived with some other things already known, and thus recognize our object as a like of those things, we can not know it at all, and it can not become a part of our store of mental experiences; while the very act by which we know it involves dissociation of it as an impression from all the impressions which it does not resemble. From this, the simplest form of knowing, to the most elaborate process of the reasoning faculty; from the recognition of single objects as like others to the recognition of classes of objects as like other classes; from the discovery of a causal relation between one set of objects or changes that is like the causal relation between another set of objects or changes to the discovery of the causal likeness connecting great groups of objects and activities, and finally all objects and changes whatsoever, there is throughout the same process at work—the process of the association of likes and the dissociation of unlikes—that conditions the mode of all our mental operations. So-called thought or reflection, for example, is simply the recovery into consciousness, for the purposes of knowledge, of cognitions more or less simple, more or less complex; cognitions recoverable as images, symbols, or terms from the classes in which they have been first associated by the mind; and the sense of pleasure felt by a thinker in discovering analogies can spring only from satisfaction of the demand, even in mental processes, that likes shall be brought together and unlikes separated. Classification in all its forms, whether in the ordinary business of life, as a means to scientific investigation, or for the ends of philosophic thought, illustrates the same necessity: at first unlikes and likes are mingled indiscriminately, and at first the mind regards them, if roughly, as being alike; but a sense of unrest leads to further examination of the aggregated elements until, by a more con-
