In the first place, we have to note that the hackneyed expression, “transformation” of pleasure into pain, and _vice versâ_, is inaccurate. Pain cannot be changed into pleasure or pleasure into pain, any more than black can be changed into white. What is meant is that the conditions of existence of the one disappear to give place to the conditions of existence of the other. There is succession, but not transformation; a symptom does not transform itself into its opposite. This succession, abrupt or gradual, leads us to ask whether there might not be a common basis, a certain identity of nature, between the two antagonistic phenomena. The question thus put may be answered by one of two alternative hypotheses. 1. The admission that the difference is fundamental and irreducible, that pain is as clearly to be distinguished from pleasure as the visual sensation is from the auditory sensation: that these feelings constitute an antinomy—an irreconcilable antagonism. The clearest affirmation of this thesis is found in those writers who make pleasure and pain “sensations” comparable to other sensations, and having their own specific character. 2. The admission that the difference is one of degree, not of nature; that the two contrary manifestations are only two _moments_ of the same process; that they differ from each other only as sound differs from noise, or a very acute sound from a very deep one, both resulting from the same cause—the number of vibrations in any given space of time. I am myself inclined to maintain this second hypothesis. Let us take as an example a simple case where the process is manifested in its totality. We have a person in a so-called indifferent, neutral, or medium state, that is to say, one which cannot be described as agreeable or painful; the individual is simply alive, that is all. He is sensitive to the perfume of flowers; some are placed in his room—pleasure is the result. At the end of an hour all is changed; the subject is incommoded by the smell of the flowers, and avoids them. Hence we have three successive moments: indifference, pleasure, pain.