This being the case, it is readily apparent that the various qualities of muscular movement,—languid, intentional, unintentional, or deliberate,—have one and all a strictly quantitative basis. If a man’s smile spreads out into a grin, this change in the quality of his countenance is caused by the simple addition of fully functioning neuro-muscular units under the skin of his face. If the grin dwindles to the smile, and that again vanishes into a look only faintly reminiscent of pleasure, the opposite process of subtraction is then taking place. Consider now the further implications of this law of Lucas and Adrian. For the logical inference is that when we merely _think_ of lifting our arm, but do not lift it visibly, an essential part of the arm-lifting mechanism has been nevertheless specifically stimulated to activity; it is only because not enough muscle fibers have been innervated to overcome the inertia of the limb that the arm does not rise. That is, indeed, our common experience. When we lie abed on a cold winter morning and speculate on the question of getting up, our imagination of the heroic deed is perfectly of a piece with the real business of shivering on the drafty floor. Indeed, according to the all-or-none principle, since even the slightest neuro-muscular activity is _positive_, the terms “overt” or “covert” as applied to action refer merely to what an observer can or cannot readily perceive. This principle can be justly applied to the problem of the meaning of words. The implication is sound that even though the action-patterns which are aroused by words are unfelt by us or invisible to an onlooker, these motor tendencies are just as truly positive physiological events, so far as they go, as are the most violent efforts we openly manifest. For the pattern of action is the same; the only difference is in the number of nerve and muscle fibers involved.