In the progress of human knowledge art precedes science. The first efforts of man are practical. He has an object in view, and tries various means of accomplishing it. Some of these utterly fail, some succeed imperfectly, and others are effectual, but at an unnecessary expense of time and trouble. As his experience increases, he gradually lays down for himself certain practical rules. If the business in which he is engaged can be managed by a solitary individual, these rules may be known only to himself, and be lost at his death. It is thus that we have lost many of the secrets of the ancient painters. But if it be one that requires co-operation, they become known to his assistants and to his pupils, and gradually to all who are engaged in similar pursuits. Many minds are employed in improving them and in adding to their number, until at length they swell into a system. It may be long, however, before they exist in any but a traditional form. The great architects of the middle ages left behind them no written precepts. They taught their pupils by oral instruction, and the rest of the world and posterity by example. The desire, however, to communicate and perpetuate information is one of the strongest passions of inventive minds. As books multiply and become the principal means by which this can be effected, those who are conscious of superior knowledge become writers. They compose treatises in which the means which are supposed to be productive of certain effects are arranged and preserved; and the knowledge which previously rested on individual experience or traditional routine becomes an art.