"The study of nature is not a luxury, but a necessity for a happy life."

"The pleasure of the soul is superior to that of the body."

"Justice is a contract of utility entered into to prevent men from harming or being harmed by one another."

"We must therefore pursue the things that make for happiness, seeing that when happiness is present, we have everything; but when it is absent, we do everything to possess it."

"No pleasure is a bad thing in itself; but the means by which certain pleasures are gained bring troubles many times greater than the pleasures."