The happiness of your life depends upon the quality of your thoughts.

Marcus Aurelius wasn’t a self-help guru—he was a Roman emperor trying to stay sane while running an empire. Meditations was his way of thinking out loud, sorting through anger, fear, and frustration so they didn’t control him. His point here is simple: your thoughts are inputs, and your life is the output. Garbage in, garbage out.

He wasn’t promoting blind optimism. He was arguing for discipline—mental hygiene. If you don’t manage what you think about, your mind fills with noise, and your emotions follow. Train your thoughts, and you start to train your reactions, your habits, and eventually your life.