Just as iron rusts from disuse... even so does inaction spoil the intellect.

A man was desired to rise from bed because the sun was already up. He replied: "If I had as far to go and as much to do as he has, I should be up by now; but having but a little way to go, I shall not get up yet."

Weight is caused by one element being situated in another; and it moves by the shortest line towards its centre, not by its own choice, not because the centre draws it to itself, but because the other intervening element cannot withstand it.

Perspective is nothing more than a rational demonstration applied to the consideration of how objects in front of the eye transmit their image to it, by means of a pyramid of lines. The Pyramid is the name I apply to the lines which, starting from the surface and edges of each object, converge from a distance and meet in a single point.

Whoever in debate quotes authority uses not intellect, but memory.

The boundaries of bodies are the least of all things.

The eye transmits its own image through the air to all the objects which face it, and also receives them on its own surface, whence the "sensus communis" takes them and considers them.