Art is never finished, only abandoned.

I love those who can smile in trouble, who can gather strength from distress, and grow brave by reflection. 'Tis the business of little minds to shrink, but they whose heart is firm, and whose conscience approves their conduct, will pursue their principles unto death.

There is no result in nature without a cause; understand the cause and you will have no need of the experiment.

To speak of this subject you must... explain the nature of the resistance of the air, in the second the anatomy of the bird and its wings, in the third the method of working the wings in their various movements, in the fourth the power of the wings and the tail when the wings are not being moved and when the wind is favourable to serve as guide in various movements.

O painter skilled in anatomy, beware lest the undue prominence of the bones, sinews and muscles cause you to become a wooden painter from the desire to make your nude figures reveal all.

I awoke, only to see that the rest of the world is still asleep.

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.