Institutio Oratoria

Otherwise there will be nothing by which we can prove anything; there must be something which either is or is believed to be true, by means of which doubtful things may be rendered credible. We may regard as certainties, first, those things which we perceive by the senses, things for instance that we hear or see, such as signs or indications; secondly, those things about which there is general agreement, such as the existence of the gods or the duty of loving one's parents;

thirdly, those things which are established by law or have passed into current usage, if not throughout the whole world, at any rate in the nation or state where the case is being pleaded—there are for instance many rights which rest not on law, but on custom; finally, there are the things which are admitted by either party, and whatever has already been proved or is not disputed by our adversary.

Thus for instance it may be argued that since the world is governed by providence, the state should similarly be governed by some controlling power: it follows that the state must be so governed, once it is clear that the world is governed by providence.

Further, the man who is to handle arguments correctly must know the nature and meaning of everything and their usual effects. For it is thus that we arrive at probable arguments or εἰκότα as the Greeks call them.

With regard to credibility there are three degrees. First, the highest, based on what usually happens, as for instance the assumption that children are loved by