Preferred indifferent
The Stoic concept of a preferred indifferent is unique among Hellenistic philosophies such as Epicureanism and Cynicism.
The Stoics advocated living in accordance with Nature. In this sense, that which was “preferred” was whatever would help people live in accordance with Nature. On the contrary, that which was not preferred was whatever prevented living in accordance with nature.
Things such as life, health, beauty, wealth and reputation were preferred. Things like ugliness, poverty, disease, death and ignobility were not preferred. However, they were all “indifferent” since the Stoics realised that possessing these things itself could not make a person—or prevent them—from living a virtuous life. Rather it was how these were used that made a difference.
The ingenuity of this idea lies in the Stoic’s recognition that some things were in fact “preferred” by the human mind and body and that it was perfectly normal—like wealth or beauty. The importance was in realising that as people we ought to be indifferent to these: losing them ought not make us less virtuous. The opposite was also true, as Seneca points out in Letter 66:
A great man can come from a humble hut; an attractive and great mind can come even from an ugly and modest body. And so I think that nature produces certain such people just to confirm that virtue can come to exist in any place. If she were able to create naked minds she would have done so; now she does something better. She creates certain people who are physically impeded but who nevertheless break through the obstacles.
This was an important recognition to not judge a book by its cover or—to stretch a metaphor—if you were the book, to not be let down by your own cover.