So obviously and blatantly real is the world around us—our own bodies and all things within reach of our senses—that from time immemorable man has tended to give it exclusive status—to feel that things in and related by space are the only realities. Such a belief is adequate for all practical affairs, but philosophers have always recognized that something is lacking in it. The very statement of the existence and reality of the external world brings in a second element: the mind that recognizes and admits it; the agent that by its own consciousness establishes the contrast of internal and external. This mind—the group of all our interrelated mental faculties, conceptions, and perceptions, and our consciousness of them—is definitely not something external, not something in space that can be handled physically, and studied in its mechanical and geometrical relationships. It is completely invisible, intangible, and inaudible, and obviously we neither taste nor smell it. It forms no part of the external world, and yet the very fact that we are conscious of the world is a tacit recognition of the mind that perceives it. An inevitable dualism seems to result between the internal mind and the external world it perceives, between the subjective and the objective.