The Academy would become the center of Greek learning for almost a millennium. Plato can be understood as idealistic and rationalistic, much like Pythagorus but much less mystical. This is ultimate reality, permanent, eternal, spiritual. On the other hand, there s phenomena, which is a manifestation of the ideal. Phenomena are illusions which decay and die.
Ideals are unchanging, perfect.
They only approximate that perfect triangle, the ideal triangle. If it seems strange to talk about ideas or ideals as somehow more real than the world of our experiences, consider science. Ideas are available to us through thought, while phenomena are available to us through our senses. So, naturally, thought is a vastly superior means to get to the truth. This is what makes Plato a rationalist, as opposed to an empiricist in epistemology.