It is this feeling of the relation of certain states of mind to certain other states of mind, which solves the whole mystery of mental analysis, that seemed at first so inexplicable,--the virtual _decomposition_, in our thought, of what is by its very nature, _indivisible_. The mind, indeed, it must be allowed, is absolutely simple in all its states; every separate state or affection of it must therefore, be absolutely simple; but in certain cases, in which a feeling is the result of other feelings preceding it, it is its very nature to appear to involve the union of those preceding feelings; and to distinguish the separate sensations, or thoughts, or emotions, of which, on reflection, it thus seems to be comprehensive, is to perform an intellectual process, which, though not a real analysis, is an analysis at least relatively to our conception. It may still, indeed, be said with truth, that the different feelings,--the states or affections of mind which we term _complex_,--are absolutely _simple_ and _indivisible_, as much as the feelings or affections of mind which we term simple. Of this there can be no doubt. But the complexity with which alone we are concerned is not _absolute_ but _relative_,--a seeming complexity, which is involved in the very feeling of relation of every sort. That we are thus impressed with certain feelings of relation of conceptions to conceptions, no one can doubt who knows, that all science has its origin in these very feelings; and equivalence, or equality, is one of those relations, which, from its very constitution, it would be as impossible for the mind in certain circumstances, not to feel, as it would be impossible for it, in certain other circumstances, not to have those simple feelings which it compares. With perfect organs of vision, and in the full light of day, it is not possible for us to look on a tree, or a rock, without perceiving it; but it is not more possible for us to form a conception of two trees, without regarding this state of mind, simple though it truly is, when absolutely considered as virtually involving, or as equal to, two of those separate feelings, which constituted the conception of a single tree.