The painter who draws merely by practice and by eye, without any reason, is like a mirror which copies every thing placed in front of it without being conscious of their existence.

Those who are enamoured of practice without science are like a pilot who goes into a ship without rudder or compass and never has any certainty of where he is going. Practice should always be based upon a sound knowledge of theory.

Truth was always but the daughter of time.

Nature is so delightful and abundant in its variations that among trees of the same kind there would not be found one which nearly resembles another, and not only the plants as a whole, but among their branches, leaves, and fruit, will not be found one which is precisely like another.

My body is not a tomb for animals.

A painter should begin every canvas with a wash of black, because all things in nature are dark except where exposed by the light.

One day the world will look upon research upon animals as it now looks upon research on human beings.