object in front of a mirror, when compared with its image seen in
the mirror. For the first is something in itself, and the other
nothingness.--Folks little indebted to Nature, since it is only by
chance that they wear the human form and without it I might class
them with the herds of beasts.

12.

Many will think they may reasonably blame me by alleging that my
proofs are opposed to the authority of certain men held in the
highest reverence by their inexperienced judgments; not considering
that my works are the issue of pure and simple experience, who is
the one true mistress. These rules are sufficient to enable you to
know the true from the false--and this aids men to look only for
things that are possible and with due moderation--and not to wrap
yourself in ignorance, a thing which can have no good result, so
that in despair you would give yourself up to melancholy.

13.

Among all the studies of natural causes and reasons Light chiefly
delights the beholder; and among the great features of Mathematics
the certainty of its demonstrations is what preeminently (tends to)
elevate the mind of the investigator. Perspective, therefore, must
be preferred to all the discourses and systems of human learning. In
this branch [of science] the beam of light is explained on those
methods of demonstration which form the glory not so much of
Mathematics as of Physics and are graced with the flowers of both
[Footnote: 5. Such of Leonardo's notes on Optics or on Perspective
as bear exclusively on Mathematics or Physics could not be included