Note 14 — Chapter V · GENERAL DIFFERENCES OF THE TWO LIVES WITH RESPECT TO HABIT.

Pleasure and pain are always absolute sensations, but they may depend upon relative circumstances; that degree of cold, for example, does not incommode the inhabitant of Spitzbergen, which would be very painful to a man from a temperate climate. In order to understand how habit produces these effects, we must recollect that the repetition of the same sensations on the same part exhausts at length the sensibility of it. Hence we may conceive how the contact of a body upon a living surface may cease to be painful, while any division or solution of continuity of one of our organs will be always more or less so, because the nerves that are divided are unaccustomed to this sensation, and still possess their whole sensibility. The sense of sight furnishes us with a striking example of sensibility being exhausted by the continuation of the sensation; if we look for a long time with the same eye upon a white surface with a red spot in the middle of it, and then look upon a part that is all white, we shall perceive there a greenish spot; for the part of the retina which has been a long time in contact with the red rays, loses the peculiar sensibility that enables it to transmit this sensation perfectly; and of all the coloured rays which compose the white rays that now go to it, it transmits only those to which it is unacquainted; hence results the sensation of green.