Man is the metre of all things, the hand is the instrument of instruments, and the mind is the form of forms.

A common danger unites even the bitterest enemies.

All men desire by nature to know.

The body is most fully developed from thirty to thirty-five years of age, the mind at about forty-nine.

The seat of the soul and the control of voluntary movement - in fact, of nervous functions in general, - are to be sought in the heart. The brain is an organ of minor importance.

All are agreed that the various moral qualities are in a sense bestowed by nature: we are just, and capable of temperance, and brave, and possessed of the other virtues from the moment of our birth. But nevertheless we expect to find that true goodness is something different, and that the virtues in the true sense come to belong to us in another way. For even children and wild animals possess the natural dispositions, yet without Intelligence these may manifestly be harmful.

Metaphysics involves intuitive knowledge of unprovable starting-points concepts and truth and demonstrative knowledge of what follows from them.