The greater the man's soul, the deeper he loves.

He who wishes to see how the soul inhabits the body should look to see how that body uses its daily surroundings. If the dwelling is dirty and neglected, the body will be kept by its soul in the same condition, dirty and neglected.

Those who condemn the supreme certainty of mathematics feed on confusion, and can never silence the contradictions of the sophistical sciences which lead to eternal quackery.

Ask counsel of him who rules himself well.

I am not to blame for putting forward, in the course of my work on science, any general rule derived from a previous conclusion.

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.

All our knowledge is the offspring of our perceptions.