Thus for the psychologist the mental world is a system of mental objects. To be an object means of course to be object of some subject which is aware of it. What else could it mean to exist at all as object if not that it is given to some possible subject? But the world of objects is twofold; we have not only the mental objects of the psychologist but also the physical objects of the naturalist. Science must characterize the difference between those two and we pointed once before to the only fundamental difference. Physical objects are those which are possible objects of awareness for every subject; psychical objects are those which are possible objects of awareness for one subject only. The tree which I see is as physical tree object for every man, it is the same tree which you and I see; my psychical perception of the tree is object for one subject only. My perception can never be your perception. Our perceptions may agree but each has his own. As to the physical objects, we can entirely abstract from such reference to the subjects. We say simply: the tree exists or is part of nature; and only the philosopher is aware that we silently mean by it that it exists for every subject and that it is therefore not necessary to refer to any particular subject. But the perception of the tree which is either your idea or my idea evidently gets its existence only if it is referred and attached to a particular subject which is aware of it. Such subject of awareness is that which the psychologist calls consciousness and all the ideas and volitions and emotions and sensations and images which make up the mental life are then contents of the consciousness or objects of the consciousness. To have psychical existence at all means thus to be object of awareness for a consciousness. Something psychical which simply exists but is not object of consciousness is therefore an inner contradiction. Consciousness is the presupposition for the existence of the psychical objects. Psychical objects which enjoy their existence below consciousness are thus as impossible as a wooden piece of iron.