When the twenty years just past come to be looked back upon from the distant future, it is probable that their chief claim to interest will be that they saw the birth of the science of abnormal psychology. That science, inconspicuous as has been its development, has already given us a few generalizations of the first importance. Amongst such, perhaps the most valuable is that which has taught us that certain mental and physical manifestations which have usually been regarded as disease in the ordinary sense are due to the effects upon the mind of the failure to assimilate the {57} experience presented to it into a harmonious unitary personality. We have seen that the stable-minded deal with an unsatisfactory piece of experience by rejecting its significance. In certain minds such successful exclusion does not occur, and the unwelcome experience persists as an irritant, so to say, capable neither of assimilation nor rejection. Abnormal psychology discloses the fact that such minds are apt to develop the supposed diseases we have just referred to, and the fact that these and other manifestations of what we have called mental instability are the consequences of mental conflict. Now, we have already seen that a gregarious animal, unless his society is perfectly organized, must be subject to lasting and fierce conflict between experience and herd suggestion.[N] It is natural, therefore, to assume that the manifestations of mental instability are not diseases of the individual in the ordinary sense at all, but inevitable consequences of man’s biological history and exact measures of the stage now reached of his assimilation into the gregarious life. The manifestations of mental instability and disintegration were at first supposed to be of comparatively rare occurrence and limited to certain well-known “diseases,” but they are coming to be recognized over a larger and larger field, and in a great variety of phenomena. [N] The word “experience” is used here in a special sense that perhaps renders necessary a word or two of definition. The experience meant is everything that comes to the individual, not only his experience of events in the external world, but also his experience of the instinctive and often egoistic impulses at work within his own personality. 1915. Conditions which at first sight give rise to no suspicion of being acquired injuries to the mind, when they are looked at in the light of the facts we have been considering, reveal themselves as being scars inflicted by conflict as certainly as are some {58} forms of insanity. Characteristics which pass as vices, eccentricities, defects of temper, peculiarities of disposition, come when critically examined to be explicable as minor grades of defective mental stability, although, on account of their great frequency, they have been looked upon as normal, or at any rate in the natural order of things.