When treating of pleasure and pain, we remarked that they were too exclusively attributed to intensity of excitation (excessive, it was said, for pain; moderate, for pleasure), and that its _quality_ was forgotten. Since we have to do with hypotheses as to the part played by chemical conditions in affective life, since they are the most general, and since pleasure and pain also have this character of generality, it may be permitted to hazard a conjecture. This would consist in admitting that pleasure arises either when excitement increases chemical activity in the organism, without producing toxines, or when this augmentation of activity brings about the disintegration of the normal poisons; and that pain arises either when excitement creates an environment appropriate to the formation of toxines, or when, directly and at once, it promotes their formation, either generally or locally. But I would not insist on a simple _obiter dictum_, for which I can offer no proof, thrown out merely as a suggestion with regard to a question not as yet fully examined. We have spoken throughout of the chemical modifications as coinciding with emotional changes. Are they effects or causes, or both, according to circumstances? It is clear that this question is not new to us. It is the antithesis between the psychological and physiological theories of emotion, presenting itself under another aspect; there is no occasion for discussing it a second time. CHAPTER IX. THE EXTERNAL CONDITIONS OF EMOTION _Empiric period—Pre-Darwinian period of scientific research—Examination of Darwin’s three principles—Wundt and his explanatory formulas: Innervation directly modified, Association of analogous sensations, Relations of motion with sensory representations._ The movements of the eyes, mouth, face, the upper and lower limbs and trunk, and the modifications of the voice constitute the external expression of emotion which is principally reducible to muscular action. For the last half-century this subject has been studied in works so well known that it behoves us to be very brief. I shall confine myself to indicating the actual state of the question.