Historians have found that significant intellectual advances generally occur in cities or countries that have a stable political system.
Given that ...
Significant intellectual advances generally occur in cities or countries that have a stable political system.
How can it be that ...
Plato and Aristotle made unparalleled intellectual progress during a period of great political and social unrest?
Evaluate / Goal
This isn't really paradoxical, since the general rule in the first sentence is only saying that intellectual advances generally occur in stable political systems. That's not saying that advances only occur in stable political systems. So the fact that Plato and Aristotle are examples of advances that occurred in unstable political systems isn't really contradicting the general rule. It's just an exception.
So we should probably understand our task as, (Maybe they were well-insulated from instability and so within the world of Plato's Academy it still felt like a stable political system)