most varied interpretations; by the side of the efficient cause of an event we find a thousand entangled contingencies which appear so important that to disentangle them we are as much perplexed as the savage, who, unable to discriminate between causes and coincidences, returns to drink at the well which has cured him, carefully keeping to the same hour, the same gestures, and the same finery. The reason of this is that the faculty of similarity and the faculty of contiguity do not give the distinction, necessary as it is, between resemblances and co-existences which are significant and those which are not. The causal nexus between two phenomena is not perceived as something apart and _sui generis_; it is not even perceived at all. We perceive only their relation in time and space, and it is our mind which raises a succession to the height of a causal connection, by intercalating between cause and effect something of what we ourselves feel when we voluntarily order the execution of a movement. This is not the place to inquire what are the experimental conditions in which we subject phenomena to this anthropomorphic transformation; it will suffice for us to repeat here that, in perception, a chance relation between phenomena impresses us in the same way as when it is the expression of a law. Our intellectual machine sometimes works in accord with the external law and at others makes mistakes and goes the wrong way. Then we are obliged to correct it, and to try a better adjustment, either by profounder experimenting with nature (methods of concordance, discordance, variations, &c.), or by a comparison of different judgments and arguments made into a synthesis; and this collaboration of several concordant activities ends in a conclusion which can never represent the truth, but only the probable truth. The study of the laws of the mind shows us too clearly, in fact, their fluidity with regard to the laws of nature for us not to accept probabilism. There exists no certitude--only very 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. FOOTNOTES: [Footnote 33: LANGE, _Histoire du Materialisme_, II., 2me. partie, chap. iii.]