"I have said more than once that I would give all my books to have the fame of having discovered a single useful truth for the good of mankind."

"The happiness of a fool would be unbearable."

"The universe is a plenum, in which all bodies are connected, and each affects all the others."

"There are two kinds of truths: those of reasoning and those of fact."

"The actual world is the most perfect of all possible worlds."