Scientific Reasoning Begging the Question

I. Petitio Principii: (circular reasoning, circular argument, begging the question) in general, the fallacy of assuming as a premiss a statement which has the same meaning as the conclusion.

–“What a brain! And you know how to prove things, like the big shots?
–Yeah, I have a special method for that. Ask me to prove something for you, something real hard.
–All right, prove to me that giraffes go up in elevators.
–Let’s see. Giraffes go up in elevators … because they go up in elevators.
–Good, that was great! … Suppose I asked you to prove giraffes don’t go up in elevators.
–That’s easy. I just prove the same thing, but the other way around.” Fernando Arrabal, El Cementerio de Automoviles, el Arquitecto y El

III. The reason petitio principii is considered to be a fallacy is not that the inference is invalid (because any statement is indeed equivalent to itself), but that the argument can be deceptive. A statement cannot prove itself. A premiss must have a different source of reason, ground or evidence for its truth from that of the conclusion.