Long before there was any scientific physiology people perceived that the behavior of an animal confronted by physical influences is much better viewed, that is understood, by attributing to the animal _sensations_ like our own. To that which I see, to _my_ sensations, I have to _supply mentally_ the sensations of the _animal_, which are not to be found in the province of my own sensation. This contrariety appears still more abrupt to the scientific inquirer who is investigating a nervous process by the aid of colorless abstract notions, and is required for example to add mentally to that process the sensation green. This last can actually appear as something entirely novel, and we can ask ourselves how it is that this miraculous thing is produced from chemical processes, electrical currents, and the like.[35] Psychological analysis has taught us that this surprise is unjustified, since the physicist deals with sensations in everything on which he employs himself. This analysis is also able to render it clear to us that the mental addition by analogy of sensations and complexes of sensations which at the time being are not present in the field of sense or cannot even come into it, is also daily practised by the physicist, as when for example he imagines the moon an inert heavy mass although he cannot touch the moon but only see it. The totally strange character of the intellectual situation above described is therefore an illusion. The illusion disappears when I make observations (psychologically) on my own person which are limited to the sensory sphere. Before me lies the leaf of a plant. The green (_A_) of the leaf is united with a certain optical sensation of space (_B_) and sensation of touch (_C_), with the visibility of the sun or the lamp (_D_). If the yellow (_E_) of a sodium flame takes the place of the sun, the green (_A_) will pass into brown (_F_). If the chlorophyl granules be removed,—an operation representable like the preceding one by elements,—the green (_A_) will pass into white (_G_). All these observations are _physical_ observations. But the green (_A_) is also united with a certain process on my retina. There is nothing to prevent me in principle from physically investigating this process on my own eye in exactly the same manner as in the cases previously set forth, and from reducing it to its elements _X_ _Y_ _Z_.... If this were not possible in the case of my own eye, it might be accomplished with that of another, and the gap filled out by analogy exactly as in physical investigations. Now in its dependence upon _B_ _C_ _D_ ..., _A_ is a _physical element_, in its dependence on _X_ _Y_ _Z_ ... it is a _sensation_. The green (_A_) however is not altered at all _in itself_, whether we direct our attention to the one or to the other form of dependence. _I see, therefore, no oppositions of physical and psychical, no duality, but simply identity._ In the sensory sphere of my