Lombroso’s criminal psychology deserves much more attention than it has hitherto received. In relation to the practical problems of criminal jurisprudence it is more important than were his purely anthropological investigations, although these latter have attracted far greater popular notice. Alike to the expert in forensic medicine and to the psychiatrist, criminal psychology possesses greater diagnostic importance; for it is often necessary to determine whether, in a certain individual, acquired mental infirmity, congenital mental infirmity, or some special and peculiar type of degeneracy, predominates. It is not permissible for the medical jurist to argue as follows: “This individual has remarkable physical characters and incomprehensible psychical peculiarities, and he is therefore ill and irresponsible.” The incomprehensible character of a crime or the enigmatical character of a personality is not rarely used as an argument for the belief that mental disorder, and consequent irresponsibility, exists. But this is not a valid inference. A knowledge of the psychology of the respectable bourgeois and of that of the ordinary philistine, or a limited acquaintance with the insane confined in institutions, does not provide any experience of the interweaving of complicated and peculiar motives and feelings in the psyche of the criminal nature. A thorough knowledge of criminal psychology, which is rare to-day,[25] and which, for obvious reasons, is difficult to obtain, is an essential preliminary for the distinction of criminal natures from the insane and from imbeciles. Not infrequently it is maintained that the performer of a criminal act cannot be normal if we are unable to discover any profit which he could have derived from his act. The characteristic of a criminal, it is held, is to injure others to gain a personal advantage; but to injure others simply for the sake of doing injury is said to be characteristic of psychological anomaly.