O Man, who will discern in this work of mine the wonderful works of Nature, if you think it would be a criminal thing to destroy it, reflect how much more criminal it is to take the life of a man; and if this, his external form, appears to thee marvellously constructed, remember that it is nothing as compared with the soul that dwells in that structure; for that indeed, be it what it may, is a thing divine. Leave it then to dwell in His work at His good will and pleasure, and let not your rage or malice destroy a life — for indeed, he who does not value it, does not himself deserve it.

The Notebooks of Leonardo da Vinci (Richter, 1888) - XIX Philosophical Maxims. Morals. Polemics and Speculations.