Nothing can appear to our eyes more uniform than a piece of glass; yet we know, from its composition, as a product of art, that it is a congeries of bodies, which have no similarity to each other, and which truly exist separately from each other, in the compound, as they existed separately before the composition, though the lines of space which divide them have now ceased to be visible to our weak organs; and though, instead of being composed of alkaline and siliceous matter, which we know to be different in their qualities, the beautiful transparent substance, considered by us, were, as far as we know, _simple_, in the chemical sense of the term, it would still be as truly an aggregate of many bodies, not dissimilar, indeed, as in the former case, but each similar in qualities to the aggregate itself. The aggregate, in short, is, in every case, but a name invented by ourselves; and what we term the constituent elements, are all that truly exists. To inquire into the composition of a body, is, therefore, only to inquire what these separate bodies are which we have chosen to consider as _one_, or rather which are ranked by us as one, from their apparent continuity. I have dwelt the longer on this point of the _unity_ of an aggregate mass, as derived from the mind of the observer only, and not from its constituent bodies, which are truly separate and independent of each other, and must always be separate and independent, whatever changes they may seem to undergo, in the various processes of composition and decomposition, because this is one of the most simple, and, at the same time, one of the most convincing examples of a tendency of the mind, which we shall often have occasion to remark in the course of our intellectual analysis,--the tendency to ascribe to substances without, as if existing in them like permanent physical qualities, the relations which ourselves have formed, by the mere comparison of objects with objects, and which, in themselves, as relations, are nothing more than modifications of our own mind. It is very difficult for us to believe, that, when we speak of a rock, or a mountain, or, perhaps, still more, when we speak of a single leaf or blade of grass as _one_, we speak of a plurality of independent substances, which may exist apart, as they now exist together, and which have no other unity than in our conception. It is the same with every other species of relation. The tallness of a tree, the lowness of a shrub or weed, as these relative terms are used by _us_ in opposition, do not express any real quality of the tree, or shrub, or weed, but only the fact that our mind has considered them together; all which they express, is the mere comparison that is in _us_, not any quality in the external objects; and yet we can scarcely bring ourselves to think, but that independently of this comparison, there is some quality, in the tree, which corresponds with our notion of tallness, and some opposite