Perspective through a viewing hole, with four moral maxims
A pen diagram of the eye viewing a circular wall, above red-chalk sayings
In red chalk Leonardo sets down four aphorisms: lust is the cause of generation, gluttony the maintenance of life, fear the prolonging of life, and pain the safeguard of the instrument. Below, a pen sketch demonstrates perspective, with rays running from the eye at the upper left to a square framing an inscribed circle. He explains that this shows how a circular wall equidistant from the eye is compelled to be seen through a single hole, keyed by the letters v t, r, m a n, s o t, t s and v s.
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The eye viewing a circular wall through a hole
The construction is keyed v t - r - m a n - s o t. Leonardo writes that t s is how the eye has seen a circular wall equidistant from the eye, and that the wall v s forces the perspective to be seen through a single hole.
Four maxims on lust, gluttony, fear and pain
Lust is the cause of generation; gluttony is the maintenance of life; fear or dread is the prolonging of life; and pain is the safeguard of the instrument. The four sayings form a set of moral reflections on the drives of the body.