Concerning the origin of passion, moralists and novelists have remarked that it comes into being in two different ways—by a thunderbolt or by “crystallisation,” by sudden action or by slow actions. This double origin denotes predominance either of the affective life or of the intellectual life. When passion is born suddenly it issues directly from emotion itself and retains a certain violence of nature, so much at least as its metamorphosis into a permanent disposition admits. In the other case the initiative is taken by the intellectual states (images, ideas), and the passion is slowly constituted as the result of association which itself is only an _effect_, for it obeys a latent influence, a hidden factor, an unconscious activity only revealed by its work. Representations only attract each other and associate by reason of their affective similitude, of the emotional tone which is common to them, and by successive additions these little streams form a river. This form of passion, on account of its origin, has less ardour and more tenacity. After distinguishing passion from emotion, it is still necessary to separate it from insanity, its other neighbour. Certain authors have at once classed all passions with insanity; I cannot accept this proposition. It may suit the moralist, by no means the psychologist. But the task of separation is very delicate, and cannot be attempted in this Introduction. The distinction between the normal and the morbid, always difficult, is especially so in the case of the psychology of the feelings. I shall endeavour elsewhere (Part I., Chap. iv.) to find the indications which enable us to establish this separation legitimately, and the task which we now put aside in its general form will come before us later on in the case of each particular emotion. PART I. GENERAL PSYCHOLOGY. CHAPTER I. PHYSICAL PAIN. _Its anatomical and physiological conditions; pain nerves, transmission to the centres—Modifications of the organism accompanying physical pain: circulation, respiration, nutrition, movements—Are they the effects of paint?—Pain is only a sign—The analgesias: unconsciousness of pain and intellectual consciousness—Retardation of pain after sensation—Hyperalgesia—Nature of pain: theory that it is a sensation; theory that it is a quality of sensation—Pain may result from the quality or the intensity of the stimulus—Hypotheses regarding its ultimate cause: it depends on a form of movement, a chemical modification._