The eye is the window of the human body through which it feels its way and enjoys the beauty of the world.

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.

The motive power is the cause of all life.

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.

Simplicity is the ultimate sophistication.

Though I may not . . . be able to quote other authors, I shall rely on that which is much greater and more worthy - on experience.

Although nature commences with reason and ends in experience it is necessary for us to do the opposite, that is to commence with experience and from this to proceed to investigate the reason.