I must, however, glance for a moment at fields outside my own, for the purpose not of controversy but of elucidation; not to deal with the subject matter of my neighbors, but to mark off my own more definitely. What then, I ask, is the most general expression by which to designate the singularities of the human mind, the principles on which it acts, its immutable modes of behavior, the invariants that recur amid all the complex varieties of its processes? The principal invariants are the positing of a manifold of some kind, and the apprehending of that manifold as coherent. The manifold is not given, but is posited by the mind. The positing is a mental function, just as much as the apprehending of the plurality as coherent is a mental function. The particular manifolds of space and time experience are said to be given, but they would not be received by the mind were not the function of manifold-positing prepared to apprehend them. In recent physical science the notion of the manifold plays a conspicuous rôle. Subtle speculations are employed to define the kinds of manifold which the physicist finds opportune, and the kind of unity of which these manifolds are respectively capable. The two terms mentioned are themselves the most abstract conceivable, and naturally, that which is here taken to underlie all the constructive, world-building activity of the mind in every possible direction can only be expressed in the most sublimated language. But the notions themselves, or rather the acts of the mind, the functions designated, are rich and replete with concrete utility when applied to subject matter in the different fields. Wherever we turn we find that the assurance of reality depends on the joint use of the two principles mentioned, the joint operation of the two kinds of mental action; that is to say—on the positing of a manifold and on the simultaneous apprehension of the subject matter to which it relates as coherent, as unified.