THAT PAINTING DECLINES AND DETERIORATES FROM AGE TO AGE, WHEN
PAINTERS HAVE NO OTHER STANDARD THAN PAINTING ALREADY DONE.

Hence the painter will produce pictures of small merit if he takes
for his standard the pictures of others. But if he will study from
natural objects he will bear good fruit; as was seen in the painters
after the Romans who always imitated each other and so their art
constantly declined from age to age. After these came Giotto the
Florentine who--not content with imitating the works of Cimabue his
master--being born in the mountains and in a solitude inhabited only
by goats and such beasts, and being guided by nature to his art,
began by drawing on the rocks the movements of the goats of which he
was keeper. And thus he began to draw all the animals which were to
be found in the country, and in such wise that after much study he
excelled not only all the masters of his time but all those of many
bygone ages. Afterwards this art declined again, because everyone
imitated the pictures that were already done; thus it went on from
century to century until Tomaso, of Florence, nicknamed Masaccio,
showed by his perfect works how those who take for their standard
any one but nature--the mistress of all masters--weary themselves in
vain. And, I would say about these mathematical studies that those
who only study the authorities and not the works of nature are
descendants but not sons of nature the mistress of all good authors.
Oh! how great is the folly of those who blame those who learn from
nature [Footnote 22: _lasciando stare li autori_. In this
observation we may detect an indirect evidence that Leonardo
regarded his knowledge of natural history as derived from his own
investigations, as well as his theories of perspective and optics.
Compare what he says in praise of experience (Vol II; _XIX_).],
setting aside those authorities who themselves were the disciples of
nature.