CIRCULUS IN PROBANDO, in logic, a phrase used to describe a form of argument in which the very fact which one seeks to demonstrate is used as a premise, i.e., as part of the evi dence on which the conclusion is based. This argument is one form of the fallacy known as petitio principii, "begging the ques tion." It is most common in lengthy arguments, the complicated character of which enables the speaker to make his hearers forget the data from which he began. The following duologue may serve as a simple illustration of circular argument. "He speaks with angels," said one of the Master's disciples. "How know you that ?" I asked. "He himself admits it," he replied solemnly. "But suppose he lies?" I persisted. "What ! " he exclaimed, "a man who speaks with angels capable of telling a lie?" (See FALLACY.)