Necessity is the mistress and guardian of Nature.

For once you have tasted flight you will walk the Earth with your eyes turned skywards, for there you have been and there you will long to return.

The time will come when men such as I will look upon the murder of animals as they now look on the murder of men.

Nothing should be so greatly feared as empty fame.

The boundaries of bodies are the least of all things.

There is no higher or lower knowledge, but one only, flowing out of experimentation.

Being willing is not enough. We must do.

In the days of thy youth seek to obtain that which shall compensate the losses of thy old age.

I have discovered that a screw-shaped device such as this, if it is well made from starched linen, will rise in the air if turned quickly.

Men of genius sometimes accomplish most when they work least, for they are thinking out inventions.

Supreme happiness will be the greatest cause of misery, and the perfection of wisdom the occasion of folly.

Perspective is a most subtle discovery in mathematical studies, for by means of lines it causes to appear distant that which is near, and large that which is small.

The fame of the rich man dies with him; the fame of the treasure, and not of the man who possessed it, remains.

The most beautiful words of love are told in silence for a look.

The divisions of Perspective are 3, as used in drawing; of these, the first includes the diminution in size of opaque objects; the second treats of the diminution and loss of outline in such opaque objects; the third, of the diminution and loss of colour at long distances.

I have solved what color is, however ; I still have no idea about what line is.

Let not your rage or malice destroy a life.

Thou, O God, dost sell us all good things at the price of labour.

Learning acquired in youth arrests the evil of old age; and if you understand that old age has wisdom for its food, you will so conduct yourself in youth that your old age will not lack for nourishment.

Inaction saps the vigor of the mind.