Institutio Oratoria

It is, in fact, as natural to do this as to harden the muscles, increase our strength and improve our complexion by means of exercise. It is for this reason that among all nations one man is regarded as more eloquent and more attractive in his style than another (since if this were not the case, all speakers would be equal); but the same men speak differently on different subjects and observe distinctions of character. Consequently the more effective a man's speaking, the more in accordance with the nature of eloquence will it be.