The thoughts presented gain greatly in fixity and vividness if in addition to simply expressing them in abstract form we bring ourselves face to face with the facts from which they arise. For example, I lie upon my sofa. If I close my right eye the picture represented in the accompanying cut is presented to my left eye. In a frame formed by the ridge of my eyebrow, by my nose, and by my moustache, appears a part of my body, so far as it is visible, and also the things and space about it. _My_ body differs from other human bodies—leaving out of account the fact that every vivid motory idea immediately passes into movement and that contact with it determines more perceptible changes than contact with other bodies—by the circumstance, that it is only partly seen, and, especially, is seen without a head. If I observe an element _A_ within my field of vision, and investigate its connection with another element _B_ within the same field, I go out of the domain of physics into that of physiology or psychology, if _B_, to use the apposite expression that a friend[20] of mine employed upon seeing this drawing, passes through my skin. Reflections like that for the field of vision may be made with regard to the province of touch and the perceptual domains of the other senses. [20] J. Popper of Vienna. [Illustration] X. Reference has already been made to the different character of the groups of elements that we have designated by _A B C_ ... and α β γ. As a matter of reality, when we _see_ a green tree before us, or _remember_ a green tree, that is _conceive_ a green tree to ourselves, we know right well how to distinguish these two cases. The imaged tree has a much less determinate, a much more changeable form; its green is much paler and more evanescent; and, what is of especial note, it distinctly appears in a _different_ sphere. A movement that we _propose_ to execute is always only a _conceived_ movement, and appears in a different field or sphere from that of the executed movement, which moreover always takes place where the image becomes vivid enough. The statement that the elements _A_ or α appear in a different sphere, means, if we go to the bottom of it, nothing more than that they are united with divers other elements. To this extent, accordingly, the _basal component parts_ in _A B C_ ..., α β γ ... would be _the same_ (colors, sounds, spaces, times, motory sensations, innervations ...), and only the character of their union different.