Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
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