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
in the arrangement of the _libro di pittura_ which is here presented
to the reader. They are however but few.]. But its axioms being laid
down at great length, I shall abridge them to a conclusive brevity,
arranging them on the method both of their natural order and of
mathematical demonstration; sometimes by deduction of the effects
from the causes, and sometimes arguing the causes from the effects;
adding also to my own conclusions some which, though not included in
them, may nevertheless be inferred from them. Thus, if the Lord--who
is the light of all things--vouchsafe to enlighten me, I will treat
of Light; wherefore I will divide the present work into 3 Parts