An axiomatic system is complete if every tautology is a theorem (derivable from axioms). An axiomatic system is sound if every theorem is a tautology. In propositional logic, there is no distinction between a tautology and a logically valid formula. Negation, Logical biconditional, exclusive or, tautology, and contradiction are linear functions. The fundamental definition of a tautology is in the context of propositional logic. Tautology and pleonasm are not consistently differentiated in literature. The term pleonasm is most often, however, employed as synonymous with tautology. Since every tautology is provable, the logic is complete. Charvaka deemed Vedas to be tainted by the three faults of untruth, self-contradiction, and tautology. It is the "disjunction" of these two expressions which is a tautology. Every tautology of paraconsistent logic is also a tautology of classical logic. Parallelism is not tautology, but rather a particular stylistic device. Nonsense tautology, reduplication, and absurd precision have also been used in the nonsense genre. "Survival of the fittest" is sometimes claimed to be a tautology. Kant claims that this is merely a tautology and cannot say anything about reality.