 Traditionally, Stoic philosophy is considered to have three phases: early (Zeno, Cleanthes, Chrysippus, et al.), middle (Panaetius, Posidonius, et al.), and late (Seneca, Epictetus, et al.). This periodization importantly hangs on a possible development in the philosophical psychology of the Stoics—the question of whether Panaetius and Posidonius move away from so-called psychological monism. According to psychological monism, there is no non-rational part or power of the soul. Rather, the soul is one insofar as its commanding faculty is one, and rational. According to psychological monism, motivational conflict and irrational action do not result from a ‘struggle’ between the rational and the non-rational aspects of the soul; what we call irrational must be understood as (bad) states of the rational soul. Psychological monism is thus a counterposition to Plato’s and Aristotle’s account of the soul, and has major implications for the theory of action, ethics, and the theory of emotion. It is a difficult question whether middle Stoicism departs from psychological monism. The view that it did was for a long time widely accepted. However, this traditional picture has recently been contested in influential studies (Cooper 1999; Tieleman 2003). Perhaps early and middle Stoicism are more in agreement than it was previously thought. Accordingly, some recent studies of Seneca proceed on the assumption that we need not attempt to figure out whether Roman Stoicism agrees with monistic early or with pluralistic middle Stoicism (Inwood 2005).