  Introducing the idea of unnoticed oscillation of a single, partless mind is highly ingenious and must have been dialectically effective at least to some extent. However, the theory of the soul that we find in classical Stoicism appears to be committed to the view that in the case of adult humans, there simply are no motivational factors that do not depend on reason and that can significantly affect, often for the worse, how a person behaves and how their life goes. It must have been difficult to defend this view against the Platonic-Aristotelian position. And so it is not surprising that in an environment in which interest in Plato's and Aristotle's writings was on the rise again, at least one prominent Stoic philosopher, Posidonius (first century B.C.), apparently gave up at least part of the classical Stoic theory. The evidence that we have is not easy to interpret, but it very much appears that Posidonius introduced into a basically Stoic psychological framework the idea that even the minds of adult humans include, to put things cautiously, motivationally relevant forces (of two kinds) that do not depend on assent or reason at all and that are not fully subject to rational control.  (For detailed discussion, cf. Cooper 1998, 77-111.)