Institutio Oratoria

For he cannot have a knowledge of all causes, and yet he should be able to speak on all. On what then will he speak? On those which he has studied. Similarly as regards the arts, he will study those concerning which he has to speak, as occasion may demand, and will speak on those which he has studied.

What then?—I am asked—will not a builder speak better on the subject of building and a musician on music? Certainly, if the orator does not know what is the question at issue. Even an illiterate peasant who is a party to a suit will speak better on behalf of his case than an orator who does not know what the subject in dispute may be. But on the other hand if the orator receive instruction from the builder or the musician, he will put forward what he has thus learned better than either, just as he will plead a case better than his client, once he has been instructed in it.