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 pain?—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. CHAPTER II. MORAL PAIN 42 Identity of all the forms of pain—Evolution of moral pain: (1) the pure result of memory; (2) connected with representations; positive form, negative form; (3) connected with concepts—Its external study; physical signs—Therapeutics—Conclusions—A typical case of hypochondriasis. CHAPTER III. PLEASURE 48 Subject little studied—Is Pleasure a sensation or a quality?—Its physical concomitants: circulation, respiration, movements—Pleasure, like pain, is separable: physical and moral anhedonia—Identity of the different forms of pleasure—The alleged transformation of pleasure into pain—Common ground of the two states—Hypothesis of a difference in kind and in degree—Simultaneity of two opposite processes: what falls under consciousness is the result of a difference—Physiological facts in support of the above. CHAPTER IV. MORBID PLEASURES AND PAINS 61 Utility of the pathological method—Search for a criterion of the morbid state; abnormal reaction through excess or defect; apparent disproportion between cause and effect; chronicity—I. Morbid pleasures, not peculiar to advanced civilisation—Different attempts at explanation—This state cannot be explained by normal psychology: it is the rudimentary form of the suicidal tendency—Classification—Semi-pathological pleasures: those destructive of the individual, those destructive of the social order—II. Abnormal pains—Melancholic type—Whence does the painful state arise in its permanent form? from an organic disposition? or from a fixed idea?—Examples of the two cases. CHAPTER V. THE NEUTRAL STATES 73 Two methods of study—Affirmative thesis founded on observation, deduction, and psycho-physics—Negative thesis: the psychological trinity; confusion between consciousness and introspection—Diversity of temperaments.