Chapter 2 of the excellent Cambridge Companion to the Stoics is by Christopher Gill, and it covers the post-Republican Roman period of the Stoa. Gill does much to undo the stereotype that nothing novel happened to Stoicism during this period, and that people like Seneca, Musonius, Epictetus and Marcus were simply rehashing stuff they got from the earlier, Greek period, or else bringing Stoicism to the masses, but with no further elaboration.
One of the nice things to learn from this chapter is that Marcus Aurelius actually set up four chairs of philosophy in Athens, one each for the four major schools vying for prominence at the time: Stoicism (of course), but also Epicureanism, Aristotelianism and Platonism.
The teaching of Stoicism was based on two elements: i) the availability of a by then large canon of classical texts; and ii) the solidification of the three-pronged curriculum, including the study of logic and physics, together with ethics.
In terms of originality, Gill points out that new treatises (as opposed to just textbooks) were being produced during the Roman Imperial period. For instance, Hierocles wrote his Elements of Ethics, and Cornutus wrote a Summary of the Traditions of Greek Theology. Gill counts Seneca’s Natural Questions amongst the original contributions to Stoicism, while Cleomedes wrote his Caelestia, which dealt with Stoic physics.
As Gill puts it, however, one shouldn’t expect complete originality, because that would actually signal an abandonment, or radical revision of the original doctrines. Rather:
“The relevant kind of originality is not, I take it, putting forward a completely new set of ideas, but rather making a new and significant move in a continuing debate based on an existing (Stoic) framework of thought.”
In ethics too, there is innovation. For instance, says Gill:
“Seneca offers an alternative threefold pattern of ethical guidance : (1) assessing the value of each thing, (2) adopting an appropriate and controlled impulse toward objects pursued, and (3) achieving consistency between impulse and action. Epictetus’ threefold pattern is similar but not identical.”
Perhaps the most interesting section of the chapter deals with so-called “doctrinal eclecticism,” i.e., the measure in which Roman Stoics absorbed other school’s notions into their own version of Stoicism. This is very relevant today, with talk of Modern Stoicism and discussions about just how far one could reasonably go from Zeno and Chrysippus (or even Seneca and Epictetus) and still call the result “Stoicism.”
For instance, Seneca had no trouble adopting some Epicurean notions and criticizing fellow Stoics, if that made sense to him. Musonius’ practical advise was obviously heavily influenced by Cynicism (and so was Epictetus’, the latter, according to Gill, practicing a version of “tough” Stoicism).
Turning to the Meditations, Gill notes:
“Most puzzling of all, despite his frequent adoption of a cosmic perspective on ethical life, [Marcus] sometimes expresses indifference about which worldview is correct: the Stoic providential one or the Epicurean view that the universe is a fortuitous collection of atoms. In Marcus’ case, there is no a priori reason to demand doctrinal consistency.”
The chapter also includes a detailed account of Roman Stoicism’ relations to the other schools of the time and how especially Academic (i.e., Platonic) criticism led to a reciprocal dialogue between the two philosophies. Indeed, neo-Platonists were still influenced by Stoicism even though the latter had by that time ceased to be a living philosophy. And of course, in turn what is referred to as “Middle Platonism” heavily influenced Christianity.