Hic Rhodus, hic salta
The Latin expression Hic Rhodus, hic salta originated from one of Aesop's Fables, namely The Boasting Traveller. In the original Greek, it is rendered Αὐτοῦ γὰρ καὶ Ῥόδος καὶ πήδημα.
The listener is challenged to prove his claims directly instead of relying on absent witnesses.
Another, perhaps more famous exemplary usage by Marx is in his Capital: The money-owner, who is as yet only a capitalist in larval form, must buy his commodities at their value, sell them at their value, and yet at the end of the process withdraw more value from circulation than he threw into it at the beginning. His emergence as a butterfly must, and yet must not, take place in the sphere of circulation. These are the conditions of the problem. Hic Rhodus, hic salta!*