But there is one particular difficulty which makes the psychological description so much harder than that of the physicist, and which gives rise to many disagreements and discussions in psychological literature. The psychologist has not only to tear the complex into pieces and thus to seek the elements, but he has to fixate those elements for the purpose of communication, as, of course, a scientific description demands that he be able to give account to others of what he experiences. The physicist has no difficulty whatever in that line because, as we saw, the world of physical things is the world which all men are sharing together. Every element which I find in it, I can show to every other person, and if I cannot show that particular thing, because I cannot yet carry the mountain to another place, then I can at least measure it, as we share those standards of space. Thus natural science has in its objective measurements the possibility of describing every part of the physical world. The psychical world, on the other hand, is as we saw, the world which is private property. Every effort at description is thus entirely in vain as long as our mental facts cannot somehow be linked with physical happenings. If I say that I have in my mind sweetness or sourness, or bitterness or saltness, I cannot carry any understanding to anyone else and therefore cannot give any description until I have agreed that I mean by sweetness the sensation which sugar gives me, and by saltness the sensation of salt. The sugar and salt I can point out to my neighbor and only in that way I understand what he means if he says that he tastes salt and sweet; otherwise I should have no means whatever to discriminate whether that which he calls a sweet taste sensation is not just what I call headache. Where no such direct relation for a physical thing is known, description of the mental element would remain impossible. Of course, every perception of the outer world, all our seeing and hearing, and touching and tasting, offers us at once such definite connection between the inner experience and a piece of the physical universe. Our own organism is also such a piece of physical nature: just as I describe my tasting or touching, I may describe the perception of my arms and legs or my inner organs. Thus everything which is material of perception gives us a handle for a real psychological description. Psychology usually calls the elements of these perceptions sensations. Whatever is composed of sensations is thus describable.