1. Argument. Questions that have arisen about the “existence” of elementary particles and other entities of physics have often been dismissed as unprofitable, with the tacit assumption that the categories suitable for the discussion of everyday knowledge are not suitable for the discussion of physical knowledge, which requires mathematical treatment. But for the layman who stumbles at the discontinuity between his world and that of mathematical physics, and for the physicist who wishes his knowledge of the world to have some kind of unity, a scheme which will embrace both kinds of knowledge is essential. Such a scheme is developed from a consideration of the dependence on mind, and conceptual modes of thought, common to science and all other intellectual approaches to the world, which is seen clearly in the growth of science from more primitive concepts. It begins with the concept as the most elementary category of the organization of experience, and moves to the construct, a distinctly scientific category arising out of the concept, and the isolate, corresponding to elements of reality not directly perceived. An escape from certain paradoxes generated by the incomplete nature of perceptual experience is thus provided.