who are not in sympathy with his studies, and his brain should have the power of adapting itself to the tenor of the objects which present themselves before it, and he should be freed from all other cares. And if, while considering and examining one subject, a second should intervene, as happens when an object occupies the mind, he ought to decide which of these subjects presents greater difficulties in investigation, and follow that until it becomes entirely clear, and afterwards pursue the investigation of the other. And above all he should keep his mind as clear as the surface of a mirror, which becomes changed to as many different colours as are those of the objects within it, and his companions should resemble him in a taste for these studies; and if he fail to find any such, he should accustom himself to be alone in his investigations, for in the end he will find no more profitable companionship.
Leonardo.
LI
If you are fond of copying other Men's Work, as being Originals more constant to be seen and imitated than any living Object, I should rather advise to copy anything moderately carved than excellently painted: For by imitating a Picture, we only habituate our Hand to take a mere Resemblance; whereas by drawing from a carved Original, we learn not only to take this Resemblance, but also the true Lights.
Leon Battista Alberti.
LII
There are a thousand proofs that the old masters and all good painters from Raphael onwards executed their frescoes from cartoons and their little easel pictures from more or less finished drawings.... Your model gives you exactly what you want to paint neither in character of drawing nor in colour, but at the same time you cannot do without him.