Letourneau, _Physiologie des Passions_, liv. i. Chap. I. ----- If we seek the special mark of passion and its characteristics among the phenomena of the affective life, we must distinguish it from emotion on one side and insanity on the other; for it is situated midway between the two. It is difficult to express with clearness and precision the difference between emotion and passion. Is it a difference of nature? No, for emotion is the source whence passion flows. Is it a difference of degree? This distinction is precarious, for while there are calm emotions and violent passions, the contrary may also be met with. A third difference remains, duration. It is generally said that passion is an enduring state; emotion is the acute form, passion the chronic form. Violence and duration are the characters usually assigned to it; but we may further define its essential nature. _Passion is in the affective order what an imperative idea (idée fixe) is in the intellectual order_; we might add what a contraction is in the motor order. It is the affective equivalent of the imperative idea. This needs some explanation. The normal intellectual state is a plurality of states of consciousness determined by the mechanism of association. If at a given moment a perception or representation arises and occupies alone the chief field of consciousness, ruling as a sovereign, making a space around it, and only permitting associations which are in direct relation with itself, we have a state of attention. This “monoideïsm” is by its nature exceptional and transitory. If it does not change its object, persisting or repeating itself constantly, we have the fixed idea, which may be called permanent attention. It is not necessarily morbid, as Newton’s celebrated phrase and other evidence show; but the latent or actual sovereignty of the fixed idea is absolute and tyrannical. In the same way the normal state of feeling is the succession of pleasures, troubles, desires, whims, etc., which in their temperate form, and often dulled by repetition, constitute the prosaic round of ordinary life. At a given moment some circumstance causes a shock; that is emotion. Some tendency annihilates all the others, momentarily confiscating the whole activity to its profit; that is the equivalent of attention. Usually this passage of movements in a single direction is not enduring; but if, instead of disappearing, the emotion remains fixed, or repeats itself incessantly, always the same, with the slight modifications involved in passing from the acute to the chronic stage—that is passion, which is permanent emotion. In spite of apparent eclipses, it is always ready to appear, absolute and tyrannical.