The progress of discovery has forced upon us a subdivision of the sciences into two classes, unknown in the middle ages; in one of which we discover causes; in the other, in which we are unable to find causes, we rest content with classification by species and genera. In the former we discover antecedents, in the latter types.(257) But in mediæval science, as in Greek, the latter class was regarded as the sole form of all perfect science. Hence the reason will appear why the question as to the true nature of genera and species had a monopoly of the field of inquiry; and also why the theory of predication was exalted into the most important part of logic.(258) Those who thought that genera had a real existence as essences apart from man’s mind and from nature, were denominated Realists: those who denied to them any real existence, and considered them to be a common quality labelled by a common name, were Nominalists: those who held the intermediate view, and assumed them to exist, not only as artificial names but also as general classes in the human mind, were Conceptualists. With the realist, classification was not arbitrary, but true and determined for man. With the nominalist and conceptualist it was created by man, and amenable to correction. The question, though now relegated from metaphysical to physical science, has still sufficient importance to enable us to perceive likewise the reason why these different theories could be the means of dividing men into parties. The bitterness with which a zoological inquiry of analogous character into the perpetuity of natural species(259) has been lately assailed may enable us to realize the earnestness shown on this point in the middle ages. The question, as viewed by the schoolmen, was really the fundamental one as respects knowledge; and the opinions on it are the counterpart to those which relate to the tests of truth and the nature of being in modern metaphysics. The spirit of realism was essentially the spirit of dogmatism, the disposition to pronounce that truth was already known.(260) Nominalism was essentially the spirit of progress, of inquiry, of criticism. Realism was in spirit deductive, starting from accepted dogmas: Nominalism was in spirit, though not in form, inductive. It tested classifications, and admitted opportunities for the existence of doubt. “Believe that you may know,” was the expression of the former: “Know that you may believe,” that of the latter.(261)