The whole aim of science is to find out facts, and to work out a satisfactory theory of them. Still, a theory does not necessarily lose its utility by not being altogether true. It must be intelligible and diagrammatical, or it has no title to the name theory. The facts to which it refers are not necessarily facts of experience; they may be relations of pure mathematical forms. A theory is properly a result of systematic scientific consideration, not of mere casual suggestions; and thus the word bears a somewhat eulogistic implication in contrast to ‘view.’ Theory is opposed to fact; the latter meaning, in this connection, that which is forced upon us by perception; while theory is the part of science which is contributed by the intellect and confirmed by experiment. Theory is also opposed to practice; because a theory is a scientific product, and a pure, or theoretical, theory has regard to science alone, and is often in conflict with the practical theory, which ought preferably to be the guide of immediate action. But the latter is as truly a theory as the former, and ought equally (when practicable) to be a product of scientific examination.