Once your faith persuades you to believe what your intelligence declares absurd, beware, lest you likewise sacrifice your reason in the conduct of your life.
So long as the people do not care to exercise their freedom, those who wish to tyrannize will do so; for tyrants are active and ardent, and will devote themselves in the name of any number of gods, religious and otherwise, to put shackles upon sleeping men.
Before receiving your instruction, I must tell you what happened to me one day. I had just had a closet built at the end of my garden. I heard a mole arguing with a cockchafer; 'Here's a fine structure,' said the mole, 'it must have been a very powerful mole who did this work.' 'You're joking,' said the cockchafer; 'it's a cockchafer full of genius who is the architect of this building.' From that moment I resolved never to argue.
It is with books as with the fires of our grates, everybody borrows a light from his neighbor to kindle his own, which in turn is communicated to others, and each partakes of all.
People must have renounced, it seems to me, all natural intelligence to dare to advance that animals are but animated machines.... It appears to me, besides, that such people can never have observed with attention the character of animals, not to have distinguished among them the different voices of need, of suffering, of joy, of pain, of love, of anger, and of all their affections. It would be very strange that they should express so well what they could not feel.
Descartes constructed as noble a road of science, from the point at which he found geometry to that to which he carried it, as Newton himself did after him. ... He carried this spirit of geometry and invention into optics, which under him became a completely new art.