Epictetus

Greek Stoic philosopher born into slavery who became a freed philosopher in Rome — whose Discourses (recorded by his student Arrian) and Enchiridion (handbook) are the most practically oriented Stoic texts — and whose core distinction (what is up to us vs. what is not) is the foundation of Stoic practice.

Made the distinction between what is 'up to us' (our judgments, desires, and responses) and what is 'not up to us' (everything external) — and argued that freedom comes entirely from mastering the former while releasing the latter — making Stoic philosophy directly applicable to anyone in any circumstance.