Book 9
11.89
The mode derived from relativity declares that a thing can never be apprehended in and by itself, but only in connexion with something else. Hence all things are unknowable. The mode resulting from hypothesis arises when people suppose that you must take the most elementary of things as of themselves entitled to credence, instead of postulating them: which is useless, because some one else will adopt the contrary hypothesis. The mode arising from reciprocal inference is found whenever that which should be confirmatory of the thing requiring to be proved itself has to borrow credit from the latter, as, for example, if anyone seeking to establish the existence of pores on the ground that emanations take place should take this (the existence of pores) as proof that there are emanations.This is what is commonly called arguing in a circle.
11.90
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.Compare Sext. Emp. Pyrrh. Hyp. ii. 185. The dogmatists assert that the sceptical arguments against demonstration are either demonstrative or non-demonstrative. If the latter, they fail to establish their point [namely, that there is no such thing as demonstration]; if the former, the Sceptics by assuming demonstration confute themselves. 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.
