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

O neglectful Nature, wherefore art thou thus partial, becoming to some of thy children a tender and benignant mother, to others a most cruel and ruthless stepmother? I see thy children given into slavery to others without ever receiving any benefit, and in lieu of any reward for the services they have done for them they are repaid by the severest punishments.

Tears come from the heart and not from the brain

A bird is an instrument working according to mathematical law, which instrument it is within the capacity of man to reproduce with all its movements, but not with a corresponding degree of strength, though it is deficient only in the power of maintaining equilibrium. We may therefore say that such an instrument constructed by man is lacking in nothing except the life of the bird, and this life must needs be supplied from that of man.

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.

And you who wish to represent by words the form of man and all the aspects of his membrification, relinquish that idea. For the more minutely you describe the more you will confine the mind of the reader, and the more you will keep him from the knowledge of the thing described. And so it is necessary to draw and to describe.

Just as eating against one's will is injurious to health, so studying without a liking for it spoils the memory, and it retains nothing it takes in.