The light dove, in free flight cutting through the air the resistance of which it feels, could get the idea that it could do even better in airless space. Likewise, Plato abandoned the world of the senses because it posed so many hindrances for the understanding, and dared to go beyond it on the wings of the ideas, in the empty space of pure understanding.

He who is cruel to animals becomes hard also in his dealings with men. We can judge the heart of a man by his treatment of animals.

But although all our knowledge begins with experience, it does not follow that it arises from experience.

The question is not so much whether there is life on Mars as whether it will continue to be possible to live on Earth

In man (as the only rational creature on earth) those natural capacities which are directed to the use of his reason are to be fully developed only in the race, not in the individual.

The only thing permanent is change.

All our knowledge begins with the senses...