CIRCOLO VIZIOSO (vicious circle). — A fallacy of reasoning into which one falls when it is necessary to prove two propositions together, one being proved by means of the other (which in turn is proved by the first); or again when any proposition is proved by means of a second whose value of certainty and truth depends on the first (which is not valid unless the first is).
For example, one falls into a vicious circle who proves the value of evidence by divine veracity, and divine veracity by evidence (Descartes). Evidently, the vicious circle, because of its openly illogical inconsistency, is admitted only in successive and spaced-out moments of thought, when in a complex logical process one forgets that what is currently intended or must be proved was previously assumed as the basis and premise of the demonstrative process. The vicious circle corresponds to one of the forms of petitio principii (αiτ ε i σ ϑ α l τ ἐ ξ ξ̇ ἀ ρ χ η ς) enunciated by Aristotle. Today the two expressions are commonly used as synonyms.