Institutio Oratoria

But it is clearly useless to take one or two cases, or even a hundred or a thousand, since their number is infinite. It is the duty of the teacher to demonstrate daily in one kind of case after another what is the natural order and connexion of the parts, so that little by little his pupils may gain the experience which will enable them to deal with other cases of the same character. For it is quite impossible to teach everything that can be accomplished by art.

For example, what painter has ever been taught to reproduce everything in nature? But once he has acquired the general principles of imitation, he will be able to copy whatever is given him. What vase-maker is there who has not succeeded in producing a vase of a type which he had never previously seen?