It is an acknowledged fact, that we perceive errors in the works of others more readily than in our own. A painter, therefore, ought to be well instructed in perspective, and acquire a perfect knowledge of the dimensions of the human body; he should also be a good architect, at least as far as concerns the outward shape of buildings, with their different parts; and where he is deficient, he ought not to neglect taking drawings from Nature.

It will be well also to have a looking-glass by him, when he paints, to look often at his work in it, which being seen the contrary way, will appear as the work of another hand, and will better shew his faults. It will be useful also to quit his work often, and take some relaxation, that his judgment may be clearer at his return; for too great application and sitting still is sometimes the cause of many gross errors.