We have seen that in composition the mind can only take cognizance of likeness or dissimilarity, or of abstract beauty among the ideas it brings together. But neither likeness nor dissimilarity secures harmony. We saw in the chapter on unity that likeness destroyed harmony or unity of membership, and that difference did not necessarily secure it, but only that particular imperfection in each of the harmonizing parts which can only be supplied by its fellow part. If, therefore, the combination made is to be harmonious, the artist must induce in each of its component parts (suppose two only, for simplicity's sake,) such imperfection as that the other shall put it right. If one of them be perfect by itself, the other will be an excrescence. Both must be faulty when separate, and each corrected by the presence of the other. If he can accomplish this, the result will be beautiful; it will be a whole, an organized body with dependent members;--he is an inventor. If not, let his separate features be as beautiful, as apposite, or as resemblant as they may, they form no whole. They are two members glued together. He is only a carpenter and joiner. Sec. 7. Imagination is the correlative conception of imperfect component parts. Now, the conceivable imperfections of any single feature are infinite. It is impossible, therefore, to fix upon a form of imperfection in the one, and try with this all the forms of imperfection of the other until one fits; but the two imperfections must be corelatively and simultaneously conceived. This is imagination, properly so called, imagination associative, the grandest mechanical power that the human intelligence possesses, and one which will appear more and more marvellous the longer we consider it. By its operation, two ideas are chosen out of an infinite mass, (for it evidently matters not whether the imperfections be conceived out of the infinite number conceivable, or selected out of a number recollected,) two ideas which are separately wrong, which together shall be right, and of whose unity, therefore, the idea must be formed at the instant they are seized, as it is only in that unity that either are good, and therefore only the _conception of that unity can prompt the preference_. Now, what is that prophetic action of mind, which, out of an infinite mass of things that cannot be tried together, seizes, at the same instant two that are fit for each other, together right; yet each disagreeable alone. Sec. 8. Material analogy with imagination. This operation of mind, so far as I can see, is absolutely inexplicable, but there is something like it in chemistry.