CIRCULUS IN PROBANDO (Lat. for "circle in proving"), 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 evidence on which the conclusion is based. This argument is one form of the fallacy known as petitio principii, " begging the question." 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. (See FALLACY.)