Who made the phrase "Ipse Dixit" famous?

"Ipse dixit" (Latin for "he said it himself") is an assertion without proof; or a dogmatic expression of opinion. The fallacy of defending a proposition by baldly asserting that it is "just how it is" distorts the argument by opting out of it entirely: the claimant declares an issue to be intrinsic, and not changeable.

Someone stating that he is an expert in the subject (whatever) and something is true "because I said so" is an example of this logical fallacy.

The Latin form of the expression comes from the Roman orator and philosopher Marcus Tullius Cicero (106–43 BC) in his theological studies "De Natura Deorum" (On the Nature of the Gods) and is his translation of the Greek expression (with the identical meaning) "autòs épha" (αὐτὸς ἔφα), an argument from authority made by the disciples of Pythagoras when appealing to the pronouncements of the master rather than to reason or evidence.