As I said, philosophy!
Plato believed that everything in nature changes, but that there is an eternal world of ideas outside of the natural world. Plato thought that each thing that we see is an approximation of some perfect idea that exists somewhere else. We cannot have true knowledge about things that change, so we cannot actually know the real world, but we can have true knowledge about things that we perceive through our reason. Thus Plato was very fond of mathematics, because it involves solely the use of reason. Plato believed that people were made up of a body that is a part of the natural world but also an immortal soul that is in contact with the world of ideas. When we are born, our soul no longer has the knowledge of that world, but through experience we jog its memory and recollect the true and perfect ideas.
I don�t think we have quite got to grips with the metaphysical world yet, or whether it even actually exists. But Plato experienced logic and reason, he was also open to other ideas outside of his pure understanding.