From a psychological standpoint our own mental life and that of our neighbor, that of the man and that of the child, that of the normal and that of the insane, that of the human being and that of the animal, are to be considered as a series of mental objects. They are to be analyzed, and to be described, and to be classified and to be explained, just as we deal with the physical objects in the outer world. How are these objects of the psychologist different from the objects of the physicist, from the pebbles on the way and the stars in the sky? There is only one fundamental difference and all other differences result from it. Those outer objects which we call physical, are objects for everybody. The star which I see is conceived as the same star which you see, the table which I touch is the table which you may grasp, too. But every psychical object is an object for one particular person only. My visual impression of the star, that is, my optical perception, is a content of my own consciousness only, and your impression of the star can be a content of your consciousness only. We both may mean the same by our ideas, but I can never have your perception and you can never have my perception. My ideas are enclosed in my mind. I may awaken in your mind ideas which have the same purpose and meaning, but they are new copies in your mind. We both may be angry, but your anger can never be my anger, and your volitions can never enter my mind. Every possible psychical fact thus exists in one consciousness only, while every physical fact exists for every possible consciousness.