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 varied degrees of probability. Daily practice contents itself with a very low degree of probability; judicial logic demands a rather higher one, especially when it is a question of depriving one of our fellow-creatures of liberty or life. Science claims one higher still. But there is never anything but differences of degrees in probability and conjecture.

This, then, is the definition of psychology that we propose. It studies a certain number of laws which we term mental, in opposition to those of external nature, from which they differ, but which, properly speaking, do not deserve the qualification of mental, since they are—or at least the best known of them are—laws of the images, and the images are material elements. Although it may seem absolutely paradoxical, psychology is a science of matter—the science of a part of matter which has the property of preadaptation.