HOW THAT DILIGENCE [ACCURACY] SHOULD FIRST BE LEARNT RATHER THAN
RAPID EXECUTION.

If you, who draw, desire to study well and to good purpose, always
go slowly to work in your drawing; and discriminate in. the lights,
which have the highest degree of brightness, and to what extent and
likewise in the shadows, which are those that are darker than the
others and in what way they intermingle; then their masses and the
relative proportions of one to the other. And note in their
outlines, which way they tend; and which part of the lines is curved
to one side or the other, and where they are more or less
conspicuous and consequently broad or fine; and finally, that your
light and shade blend without strokes and borders [but] looking like
smoke. And when you have thus schooled your hand and your judgment
by such diligence, you will acquire rapidity before you are aware.

The artist's private life and choice of company (493-494).