The distinction has been clearly enunciated by many writers, from Aristotle downwards, but I may best render it in the words of Locke:— “If it may be doubted, whether beasts compound and enlarge their ideas that way to any degree; this I think I may be positive in, that the power of abstracting is not at all in them; and that the having of general ideas is that which puts a perfect distinction betwixt man and brutes, and is an excellency which the faculties of brutes do by no means attain to. For it is evident we observe no footsteps in them of making use of general signs for universal ideas; from which we have reason to imagine, that they have not the faculty of abstracting, or making general ideas, since they have no use of words, or any other general signs. “Nor can it be imputed to their want of fit organs to frame articulate sounds that they have no use or knowledge of general words; since many of them, we find, can fashion such sounds, and pronounce words distinctly enough, but never with any such application; and, on the other side, men, who through some defect in the organs want words, yet fail not to express their universal ideas by signs, which serve them instead of general words; a faculty which we see beasts come short in. And therefore I think we may suppose, that it is in this that the species of brutes are discriminated from men; and it is that proper difference wherein they are wholly separated, and which at last widens to so vast a distance; for if they have any ideas at all, and are not bare machines (as some would have them), we cannot deny them to have some reason. It seems evident to me, that they do some of them in certain instances reason, as that they have sense; but it is only in particular ideas, just as they received them from their senses. They are the best of them tied up within those narrow bounds, and have not (as I think) the faculty to enlarge them by any kind of abstraction.”[14] Here, then, we have stated, with all the common-sense lucidity of this great writer, what we may term the initial or basal distinction of which we are in search: it is that “proper difference” which, narrow at first as the space included between two lines of rails at their point of divergence, “at last widens to so vast a distance” as to end almost at the opposite poles of mind. For, by a continuous advance along the same line of development, the human mind is enabled to think about abstractions of its own making, which are more and more remote from the sensuous perception of concrete objects; it can unite these abstractions into an endless variety of ideal combinations; these, in turn, may become elaborated into ideal constructions of a more and more complex character; and so on until we arrive at the full powers of introspective thought with which we are each one of us directly cognisant. * * * * *