“And here lies the essential difference between Stoicism and the modern-day 'cult of optimism.' For the Stoics, the ideal state of mind was tranquility, not the excitable cheer that positive thinkers usually seem to mean when they use the word, 'happiness.' And tranquility was to be achieved not by strenuously chasing after enjoyable experiences, but by cultivating a kind of calm indifference towards one's circumstances.”