What is the central concept in Plato's political philosophy that represents eternal, unchanging realities?

The central concept in Plato's political philosophy that represents eternal, unchanging realities is known as "Forms" or "Ideas". According to Plato, these Forms are the only true reality and are unchangeable and perfect. They exist in a separate, higher realm of understanding that can only be accessed through philosophical reasoning. In the context of political philosophy, Plato applies this concept to suggest that there is a perfect form of justice, democracy, etc., and that the role of the philosopher is to understand these forms and apply them to political life.