It is needless to insist on the first case, since many old authors never cease repeating that the painful impression is the result of a stimulation which is strong, intense, violent, prolonged. On the contrary, it is necessary to remark that this exclusive affirmation cannot be applied everywhere and always. This evidently appears in the cases of hyperalgesia, which is the reason why I brought them forward. The very disagreeable sensation which is caused by a knife against the skin certainly comes from the nature rather than the intensity of the stimulus. Beaunis remarks that certain odours, tastes, and contacts are painful at once, and do not need to be intense. Did the contact of the quill which threw Weir Mitchell’s patient into a state of anguish act by its intensity? No doubt we must recognise that hyperalgesias constitute a group apart, not strictly comparable with ordinary cases; they are pathological forms, variable in degree; but the pathological is merely an exaggeration of the normal. The error of those who refer pain solely to intensity of stimulation is in considering _objective_ conditions only; they forget the part played by the sentient subjects. Pains which depend on the quality of the stimulus are especially of _subjective_ origin, because the degree of excitability in the patient’s nervous elements is the essential ruling factor. If we admit that these two conditions—intensity and quality—act one upon the other, what afterwards happens? What is the intimate nature of the pain-producing process? The hypothesis which is most natural and simple, and in agreement with the mechanical conceptions predominant in the biological sciences, would be founded on the admission that pain corresponds to a particular form of movement. On this supposition, the affective nervous road, from the periphery to the centres, would be traversed by three different kinds of movement or of molecular disturbance: the first giving birth to pure sensation, that is to say a state of knowledge, an intellectual state; the second, which may or may not be present, giving birth to pain; the third, also either present or absent, giving birth to pleasure.