Pyrrho's disciple Timon, however, advanced some intellectual arguments which, from the standpoint of Greek logic, were very hard to answer. The only logic admitted by the Greeks was deductive, and all deduction had to start, like Euclid, from general principles regarded as self-evident. Timon denied the possibility of finding such principles. Everything, therefore, will have to be proved by means of something else, and all argument will be either circular or an endless chain hanging from nothing. In either case nothing can be proved.
Bertrand Russell, The History of Western Philosophy