Periodical recurrences of the same act, under the same circumstances, favor a presumption of pathological causation; provided always that the act is referred to the psychological motive, rather than to any adventitious combination of circumstances. This is necessary to show the neuropathic cycle, or psychic periodicity of certain thoughts, feelings or impulses, without regard to external impressions; even the sexual act taking on widely different significance when performed at different times, or by different persons—as, for instance, by an epileptic, a paralytic, a drunkard, or a man of sound mind. But, while medical science ought to be called into requisition in every case of sexual crime, it ought not to be difficult for the legal practitioner, properly read in the literature of sexual psychopathology, to correctly relegate an offender of this type to this or that category of sexual perversion.