Marcus Aurelius was Emperor of Rome and wrote private notes to himself

Marcus Aurelius ruled the Roman Empire from 161 to 180 AD — one of the most powerful positions in the ancient world — and during that time wrote private notes to himself that were never intended for publication. Compiled as Meditations, they reveal a man in constant struggle with his own weaknesses: impatience, distraction, the temptation to seek approval. 'You have power over your mind, not outside events,' he wrote. 'Realise this, and you will find strength.' The notes are remarkable precisely because they show the most powerful person in the world using philosophy as a daily practice, not a performance.

Source: Aurelius, Meditations (c. 170–180 AD)