Seneca's quote

Lucius Annaeus Seneca (often known simply as Seneca; ca. 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. While he was later forced to commit suicide for alleged complicity in the Pisonian conspiracy to assassinate Nero, the last of the Julio-Claudian emperors, he may have been innocent. His father was Seneca the Elder and his elder brother was Gallio.

All cruelty springs from weakness. Seneca's Morals: Of a Happy Life, Benefits, Anger and Clemency
It is the power of the mind to be unconquerable. The Stoic Philosophy of Seneca: Essays and Letters