The mind of man does not contain reason in the sense that it is filled up and then the container closed off from the world. Our mind and bodies work as a sort of antenna for universal reason. This antenna is designed to receive the pure signal. It was evolved from our oldest ancestors to have a increasingly better ability to do so. This is the source of our preconceptions. A Stoic is one who understands that once the “signal” reaches us it is then filtered through our emotions and desires and fears until it becomes distorted. This is the cause of our misjudgments.
It is important to realize that the ancient Stoics did not have a concept of an antenna but their concepts of preconception and the way that spiritual exercises work is to help clear the path for reason are akin to the way in which a radio signal is filtered from a broadcast. One can see how things like fear, desire, anger all distort reason in the same way that signal can become distorted. Approaching the concept of universal reason and its interplay with the mind in this way helps to understand certain edge cases which come up in Stoicism. I will amend this post as I think through this further.