Intelligent study of an actual or a potential recidivist means a fairly complete investigation and is worth days of work if this be necessary. It needs a combination of the sociological, medical and psychological standpoints. We ourselves find particularly rich fields for explanation of the case in getting the history of families and of developmental conditions and in psychological examinations. The latter has been much hampered in the past by lack of practical tests, but of late these have been developed. At the present time any intelligent observer can judge something of the mental capacity of an individual by seeing his performance, under proper conditions, on a group of tests which correspond to the normal ability of the child. The well-known Binet tests, imperfect though they probably are in some respects, form an epoch-making advance in the study of feeblemindedness. We ourselves have been at much pains in the last two years in developing, with the help of a number of psychologists, a group of tests directed to the estimation of native mental ability in older and higher types of individuals. We may hope for much greater standardization of tests in the future, but, even as it now stands, there can be no doubt that just such a practical mental classification as the work with delinquents demands can be readily carried out by qualified persons. If, avoiding _a priori_ standpoints, we enter upon a study of the recidivist, we find such a considerable number of causative factors determinable that this at once precludes the idea of crime being anything like a disease entity. Indeed, one soon comes to feel that many of the set notions about crime are academic and absurd in the light of facts ascertainable by a free-minded, practical and thoroughgoing investigation of the individual cases. Crime may be the action of a Charlotte Corday or of a Jesse Pomeroy and in form, impulse and factors of underlying causation may be found to be so varied in its manifestations that many pseudo-philosophical speculations and legal pronunciamentos on the subject are readily seen to be nothing but slipshod generalizations. Quinton, a man of great experience, in his recent work says, with apparently purposeful exaggeration, that there are just as many classes as there are criminals. Mental defect is to be considered simply as one of the causes of crime, but it is a cause so obvious, so readily determinable in most cases and so certainly irremediable and provocative of recidivism and moral contagion that one of the first steps of reform in dealing with criminals ought to be directed toward this. The mental defective is suitable neither for probation, reformatory education nor punitive measures. Custodial care alone is of service and in the case of the criminalistically inclined defective the courts should directly commit and the state protect itself by permanent guardianship.