I do not know what to say in a case so surprising, so unlooked for and so novel.

I wish, my dear Kepler, that we could have a good laugh together at the extraordinary stupidity of the mob. What do you think of the foremost philosophers of this University? In spite of my oft-repeated efforts and invitations, they have refused, with the obstinacy of a glutted adder, to look at the planets or the Moon or my glass [telescope].

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

Mathematics is the language with which God has written the universe.

I would beg the wise and learned fathers [of the church] to consider with all diligence the difference which exists between matters of mere opinion and matters of demonstration.

It is very pious to say and prudent to affirm that the holy Bible can never speak untruth -- whenever its true meaning is understood. But I believe nobody will deny that it is often very abstruse, and may say things which are quite different from wha.

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