Diogenes Laertius Vitae philosophorum 9.11.90

[90] They would deny all demonstration, criterion, sign, cause, motion, the process of learning, coming into being, or that there is anything good or bad by nature. For all demonstration, say they, is constructed out of things either already proved or indemonstrable. If out of things already proved, those things too will require some demonstration, and so on ad infinitum; if out of things indemonstrable, then, whether all or some or only a single one of the steps are the subject of doubt, the whole is indemonstrable. ¹ If you think, they add, that there are some things which need no demonstration, yours must be a rare intellect, not to see that you must first have demonstration of the very fact that the things you refer to carry conviction in themselves.