Among the problems which confront the psychiatrist in arriving at a judgment upon a mental case, few are more difficult than the estimation of the patient's mental working capacity. A rough approximation may be attained by careful observation of the patient's daily reaction to his surroundings—by the attitude he adopts to the little problems of his daily life; but such is, at best, only a very vague and unsatisfactory measure. To impose simple tasks for the purpose of obtaining objective criteria upon which to base our judgment is unusually difficult, and patients who were incapable of comprehending the new task would have to be excluded from its action; and to interest the patients in such a task sufficiently to persuade them to do their best is often not possible. The estimation of the working capacity of a large proportion of patients would thus seem at first sight to be beyond our powers to ascertain.