The Stoics thought that once human beings become rational, they immediately form false beliefs about what is good and what is bad. There are no exceptions. Even the sage once had false beliefs about the value of things. The dispute among the Stoics was not about whether this happens, but was about how to explain the reasoning that results in these beliefs. The primary evidence for this dispute is Galen's discussion in On the Doctrines of Plato and Hippocrates. On the basis of this discussion and certain assumptions about how the Stoics understood impulses in the psychology of children and adults, I set out an interpretation of (i) Chrysippus' explanation of what happens at the onset of reason when human beings form false beliefs about what is good and what is bad, (ii) Posidonius' criticism of this explanation of the origin of vice, and (iii) the explanation that Posidonius himself thought was correct.
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