Some passages from Margaret R. Graver's Stoicism and Emotion:
"Deeper motivations for the Stoic position lie within the structure of their own ethical system. Goodness in Stoic thought is essentially a notion of rightness or fit. Just as in mathematics the solution to a problem is right when it is in accordance with the system of thought which is mathematics as a whole, so in Stoic ethics a circumstance or event is good when it participates in some logically coherent system. For most things in the universe, the relevant larger system is just the universe itself, for the world as a whole operates according to underlying principles of regularity which admit of no exceptions, rather as we speak of 'laws of science.' But the mind of an adult human, while it certainly participates in the world order, also has a frame of reference which is entirely its own. Because I am a rational being, capable of stating my beliefs and the reasons for what I do, there subsists right now, in relation to my mental contents, a large number of propositions or, as the Stoics term them, axiōmata. These can be interrelated in various ways: they may all be linked into a single coherent system, or (more likely) there may be some amount of internal contradiction. Thus my beliefs, actions, and affective responses are all capable of being evaluated not only in relation to universal reason but also in relation to my own harmonious or inharmonious system of belief. Such things belong to my own frame of reference: external objects or 'indifferents' do not."