The Rules of Art.
"I would chiefly recommend, that an implicit obedience to the Rules of Art, as established by the practice of the great Masters, should be exacted from the young Students. That those models, which has passed through the approbation of ages, should be considered PERFECT AND INFALLIBLE guides; as subjects for their imitation, and NOT THEIR CRITICISM.
I am confident that this is the only efficacious method of making progress in the arts; and that he who sets out with doubting, will find life finished before he becomes the master of the rudiments. For it may be laid down as a maxim, that he who begins by presuming on his own sense, has ended his studies as soon as he has commenced them."
- Sir Joshua Reynolds, Discourse One, 1769