VICIOUS CIRCLE. - An error of reasoning into which one falls when, in having to prove two propositions together, one is proved by means of the other (a by b and b in turn by a); or again when any proposition is demonstrated by means of a second whose certainty and truth depend on the first (a by b when b stands only by virtue of a).
For example, he falls into a V. c. who proves the value of evidence by divine veracity, and divine veracity by evidence (Cartesio). Evidently, the V. c., because of its manifest logical inconsistency, is admitted only in subsequent and separated moments of thought, when in a complex logical process one forgets that what is presently intended or must be demonstrated had already previously been assumed as the basis and premise of the demonstrative process. The V. c. corresponds to one of the forms of begging the question (αἰτειάθαι τὸ ἐξ ἀρχῆς) set forth by Aristotele. Today the two expressions are commonly used as synonyms.