The problems of understanding people have their roots in the reflections upon the thoughts, emotions, and deeds of others which occur in the course of ordinary human relationships. There are difficulties involved in justifying the concepts which are generated to interpret other’s actions and states of mind. The notion of a criterion is important in the problem of the knowledge of other minds because one of the major kinds of approach offered to this difficulty is what is usually termed the criteriological solution to the problem of other minds. There are three main formulations of the criteriological solution to the problem of other minds: conventional; a priori evidence; and necessary evidence. The notion of a criterion, whose origin lies in the work of Wittgenstein, has been the subject of controversy both with regard to its nature and applications to problems about other minds.