The true felicity of life is to be free from perturbations; to understand our duties towards God and man; to enjoy the present without any serious dependence upon the future. Not to amuse ourselves with either hopes or fears, but to rest satisfied with what we have, which is abundantly sufficient; for he that is so, wants nothing. ... Tranquillity is a certain equality of mind, which no condition or fortune can either exalt or depress."
from Wikipedia: Lucius Annaeus Seneca (often known simply as Seneca, or Seneca the Younger) (c. 4 BC – AD 65) was a Roman Stoic philosopher, statesman, dramatist, and in one work humorist, of the Silver Age of Latin literature. He was tutor and later advisor to emperor Nero. He was later executed by that emperor for complicity in the Pisonian conspiracy to assassinate this last of the Julio-Claudian emperors; however, he may have been innocent.