All truths are easy to understand once they are discovered; the point is to discover them.

Some, merely to contradict what I had said, did not scruple to cast doubt upon things they had seen with their own eyes again and again.

Measure what is measurable, and make measurable what is not so.

Scripture is a book about going to Heaven. It's not a book about how the heavens go.

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual.

Philosophy is written in that great book which ever lies before our eyes — I mean the universe — but we cannot understand it if we do not first learn the language and grasp the symbols, in which it is written.

When the moon is ninety degrees away from the sun it sees but half the earth illuminated (the western half). For the other (the eastern half) is enveloped in night. Hence the moon itself is illuminated less brightly from the earth, and as a result its secondary light appears fainter to us.