The problem to which I refer is this: there are thinkers who find a particular difficulty in understanding how another mind can act on one's own. They say: the world of my consciousness is a closed circle within me; so is the world of another's consciousness within him. I cannot look into the world of another's mind. How, then, do I know that he and I are in a common world? The theory according to which we can from the conscious world infer an unconscious world which never can enter consciousness, attempts to solve this difficulty as follows. The world, it says, which I have in my consciousness is the representation in me of a real world to which my consciousness has no access. In this transcendent world exist the unknown agents which cause the world in my consciousness. In it, too, exists my own real self, of which likewise I have only a representation in my consciousness. In it, lastly, exists the essential self of the fellow-man who confronts me. Whatever passes in the consciousness of my fellow-man corresponds to a reality in his transcendent essence which is independent of his consciousness. His essential nature acts in that realm which, on this theory, is equally beyond consciousness. Thus an impression is made in my consciousness which represents there what is present in another's consciousness and wholly beyond the reach of my direct awareness. Clearly the point of this theory is to add to the world accessible to my consciousness an hypothetical world which is to my immediate experience inaccessible. This is done to avoid the supposed alternative of having to say that the external world, which I regard as existing before me, is nothing but the world of my consciousness, with the absurd--solipsistic--corollary that other persons likewise exist only within my consciousness.