Now, in order to prevent these _genera_ and _species_, and all rational ideas along with them, from being precipitated into the empty abyss of words prepared for them by these philosophers, it may be proper to ask one question, viz. if we have no idea of _genera_ and _species_, or of what different things have in common or alike, that is, if the idea is nothing but the name, how is it that we know when to apply the same general name to different particulars, which on this principle can have nothing left to connect them in the mind? For example, take the words, _a white horse_. Now, say they, it is the terms which are general or common, but we have no general or abstract idea corresponding to them. But if we have no general idea of _white_, nor any general idea of _a horse_, what have we left to guide us in applying the phrase to any but the first horse, any more than in applying the terms of an unknown tongue to their respective objects? In short, what is it that ‘puts the same common name into a capacity of signifying many particulars,’ but that common nature or kind which is conceived to belong to them? Condillac says, that without general terms, there would be no general ideas; it appears to me, that without general ideas there could be no general terms. Language without this would be reduced to a heap of proper names, and we should be as completely at a loss to class any object generally from its agreement with others, or to say at sight, this is a man, this is a horse, as to know whether we should call the first man we accidentally met in the street by the name of John or Thomas. The very existence of language is alone a sufficient proof of the power of abstraction in the human mind. It is so far from being true, according to the modern philosophy, that we have neither complex nor general ideas, that, I think, it may be proved to a demonstration that we have and can have no others. I must premise, however, that I do not believe it possible ever to arrive at general or abstract ideas by beginning in Mr. Locke’s method with particular images. This faculty of abstraction is by most writers considered as a sort of artificial refinement upon our other ideas, as an excrescence no ways contained in the common impressions of things, nor necessary to the common purposes of life; and is by Mr. Locke altogether denied to be among the faculties of brutes. It is described as the ornament and top-addition of the mind of man, which proceeding from simple sensations upwards is gradually sublimed into the abstract notions of things:— ‘So from the root Springs lighter the green stalk, from thence the leaves More airy, last the bright consummate flower.’