This entire side represents Cynics and Stoics,
That's one of the factors that would dispose me to think that Raphael balanced that with the side on the left being Epicurean.
Is it possible that there is a time element with the figures outward from the center are the oldest in Greek history, with those nearer the center being ranked not only in significance but also to a degree in time? In other words, outside of the central pair and the two groupings immediately around them, are any of the figures arranged further out (to the left, right, and foreground) visibly identifiable as being significantly LATER than the relatively early Stoics and Epicureans? I would tend to think that as a thumbnail summary most people consider Plato and Aristotle to be of most significance in history, with the Stoics and Epicureans immediately following/carrying forward their differences, and that everyone else (even Socrates) was a forerunner of this great Plato-Aristotle-Stoic-Epicurean axis.