Institutio Oratoria

IV. In speaking of quality we sometimes use the word in its most general sense, which covers a number of different questions. For we enquire sometimes into the nature and form of things: as for instance whether the soul is immortal or whether god is to be conceived of in human form. Sometimes, on the other hand, the question turns on size and number, as, for instance, what is the size of the sun or whether there are more worlds than one. In all these cases we arrive at our conclusions by conjecture, yet each involves a question of quality.

Such questions are sometimes treated in deliberative themes: for example, if Caesar is deliberating whether to attack Britain, he must enquire into the nature of the Ocean, consider whether Britain is an island (a fact not then ascertained), and estimate its size and the number of troops which lie will require for the invasion. Under the same head of quality fall questions whether certain things should be done or not and certain objects sought or avoided: such topics are specially adapted for deliberative themes, but occur with some frequency in controversial themes as well, the only difference being that in the latter we deal with what is past and in the former with the future.