Musonius Rufus is the practical Stoic. Born around 25 CE into the Roman equestrian class, he taught in Rome through the reign of Nero and was exiled by him in 65 CE — banished, like Seneca's circle, on suspicion of philosophical disloyalty. He was exiled again by Vespasian around 75 CE (having initially been exempted from Vespasian's general expulsion of philosophers in 71 CE). The recurring exile is the measure of his political effect: emperors did not trouble themselves to banish men who had no effect on their students.
His most famous student was Epictetus, who would in turn teach the lineage that ended in Marcus Aurelius. Musonius's lectures, preserved in summary by his student Lucius and gathered into the Diatribai, are direct and practical. He addresses topics that no other Stoic of his time engaged: the equal education of women, the case for vegetarianism, the ethics of sexual conduct, the question of whether kings should learn philosophy, what philosophers should wear, what they should eat, how they should furnish their houses. The level of practical detail is unusual and informative.
His emphasis is on training rather than theory. In Lecture 6 (On Training) he distinguishes the training of the soul from the training of the body, then argues that both require constant askēsis — practice, repetition, discomfort accepted deliberately so that virtue becomes habit rather than effort. The line Epictetus would later make famous — philosophy is not a body of theory but an art of living — is essentially Musonius.
He was also unusual among Stoics in his prescription of voluntary austerity for daily life. Plain food. Hard work. Cold water. Coarse clothing. Sex within marriage only, for the purpose of children, as a moral discipline. The asceticism was not punitive — it was training. And it is the direct ancestor of the modern Stoic prescription of voluntary discomfort.
He died around the turn of the second century in Rome (sometime before 101/102 CE), having been recalled from his second exile. He left no great theoretical works behind, but he shaped a teacher who shaped the most influential Stoic of the second century. The Stoic tradition has rarely been propagated by books alone; it has more often been propagated, as Musonius's example proves, by teachers whose conduct was the curriculum.