I will show that Stoic writers were perfectly capable of distinguishing among character-types also according to secondary differentia that can be exhibited independently of one another and in greater or lesser degree. Just as one may observe variations in the sea floor without disregarding the fact that all of it is equally underwater, so Stoic theory defines traits of character which differentiate one individual from another even where all concerned have the same overall moral standing. Such secondary characteristics are identified even among the virtuous; more numerous, however, and also more philosophically interesting, are the character traits of the nonvirtuous— that is to say, of ordinary flawed individuals. For it is these that we have to deal with in ourselves and others we know.
In looking at these traits of character I am especially interested in two questions. First, there is a question about how traits work as causes of emotion and of actions generally. Surviving reports make it clear that at least some traits—those called nosëmata, “sicknesses,” and arrôstëmata, “infirmities”— are defined as erroneous beliefs about the value of certain objects. It is not difficult to show that where these traits are concerned, the work of the trait in determining action and feeling is a matter of the logical operation of the corresponding proposition in the agent’s practical reasoning. For another group of traits, called “proclivities,” the evidence is less clear; I hope to show, however, that an analogous interpretation in terms of belief is plausible for these as well. This is not to deny that proclivities such as irascibility may also have a material basis, say in one’s body type or mix of constitutive humors; indeed, we should expect that every trait of character will be describable in material as well as intentionalist terms. But it is as beliefs, i.e. as corresponding to particular premises in the practical syllogism, that traits exercise their determinative capacity.
Graver, Margarete, "Stoics on the Differentiation of Character" (2004). The Society for Ancient Greek Philosophy Newsletter. 333.