HOW, IN IMPORTANT WORKS, A MAN SHOULD NOT TRUST ENTIRELY TO HIS
MEMORY WITHOUT CONDESCENDING TO DRAW FROM NATURE.

Any master who should venture to boast that he could remember all
the forms and effects of nature would certainly appear to me to be
graced with extreme ignorance, inasmuch as these effects are
infinite and our memory is not extensive enough to retain them.
Hence, O! painter, beware lest the lust of gain should supplant in
you the dignity of art; for the acquisition of glory is a much
greater thing than the glory of riches. Hence, for these and other
reasons which might be given, first strive in drawing to represent
your intention to the eye by expressive forms, and the idea
originally formed in your imagination; then go on taking out or
putting in, until you have satisfied yourself. Then have living men,
draped or nude, as you may have purposed in your work, and take care
that in dimensions and size, as determined by perspective, nothing
is left in the work which is not in harmony with reason and the
effects in nature. And this will be the way to win honour in your
art.

How to acquire universality (503-506).