The concept of reality is not a scientific one. It is not the product of some piece of research like, for instance, the concept of energy or of the integral. It does not belong to some specific science; in fact, strange as this may sound, the scientists could not be less interested in its determination or definition. It is true, of course, that theorists always receive the stimulus for their investigations from reality. But so far as the actual interest of science is concerned, which finds satisfaction in the game of reducing concepts to one another, it is of no consequence whether these concepts do or do not designate realities. In either event, the cognitive process can take its course with equal vigor. The mathematician displays no less zeal in his preoccupation with ideal structures than the historian or the economist, whose interest is centered on reality. But even the latter two construct ideal cases; and in the inquiry into their general principles they work with simplifying abstractions. In the final analysis, all science is theory and all theory has unreal abstractions as its subject-matter.