The Philosophical Writings of Descartes, Volume I

So far, I have been a spectator in this theatre which is the world, but I am now about to mount the stage, and I come forward masked.

It is just as valuable to be censured by friends as it is splendid to be praised by enemies. We desire praise from those who do not know us, but from friends we want the truth.

For there is hardly any question in the sciences about which clever men have not frequently disagreed. But whenever two persons make opposite judgements about the same thing, it is certain that at least one of them is mistaken, and neither, it seems, has knowledge. For if the reasoning of one of them were certain and evident, he would be able to lay it before the other in such a way as eventually to convince his intellect as well.

And even though we have read all the arguments of Plato and Aristotle, we shall never become philosophers if we are unable to make a sound judgement on matters which come up for discussion; in this case what we would seem to have learnt would not be science but history.