Logical division is, however, a process not fully represented by any fixed and formal distribution of topics, nor yet is it equivalent to the arrangement of genera and species according to their natural affinities, as in the admirable systems of Jussieu and Cuvier. It is something much more flexible and subtle, a carrying down into the minutest detail, of that psychological law which requires, as a condition of perfect consciousness, that feelings, conceptions, judgments, and, generally speaking, all mental modes should be apprehended together with their contradictory opposites. Heracleitus had a dim perception of this truth when he taught the identity of antithetical couples, and it is more or less vividly illustrated by all Greek classic literature after him; but Socrates seems to have been the first who transformed it from a law of existence into a law of cognition; with him knowledge and ignorance, reason and passion, freedom and slavery, virtue, and vice, right and wrong (πολλῶν ὀνομάτων μορφὴ μία) were apprehended in inseparable connexion, and were employed for mutual elucidation, not only in broad masses, but also through their last subdivisions, like the delicate adjustments of light and shade on a Venetian canvas. This method of classification by graduated descent and symmetrical contrast, like the whole dialectic system of which it forms a branch, is only suited to the mental phenomena for which it was originally devised; and Hegel committed a fatal error when he applied it to explain the order of external coexistence and succession. We have already touched on the essentially subjective character of the Socratic definition, and we shall presently have to make a similar restriction in dealing with Socratic induction. With regard to the question last considered, our limits will not permit us, nor, indeed, does it fall within the scope of our present study, to pursue a vein of reflection which was never fully worked out either by the Athenian philosophers or by their modern successors, at least not in its only legitimate direction.