It has always seemed to me extreme presumptuousness on the part of those who want to make human ability the measure of what nature can and knows how to do, since, when one comes down to it, there is not one effect in nature, no matter how small, that even the most speculative minds can fully understand.

You cannot teach a person something he does not already know, you can only bring what he does know to his awareness.

To be humane, we must ever be ready to pronounce that wise, ingenious and modest statement 'I do not know'.

There are those who reason well, but they are greatly outnumbered by those who reason badly.

Oh, my dear Kepler, how I wish that we could have one hearty laugh together. Here, at Padua, is the principal professor of philosophy, whom I have repeatedly and urgently requested to look at the moon and planets through my glass, [telescope] which he pertinaciously refuses to do. Why are you not here? what shouts of laughter we should have at this glorious folly! and to hear the professor of philosophy at Pisa laboring before the grand duke with logical arguments, as if with magical incantations, to charm the new planets out of the sky.

By denying scientific principles, one may maintain any paradox.

You may force me to say what you wish; you may revile me for saying what I do. But it moves.