Which philosopher is best known for his Theory of Ideas?

Plato is best known for his Theory of Ideas, or Theory of Forms, which posits that beyond the world of sensory experiences lies a realm of abstract, perfect forms or ideals that represent the true essence of things. According to this theory, the physical world is merely a shadow or imitation of these perfect forms. For example, while we see many different individual trees, there exists an ideal form of "tree-ness" that embodies the perfect characteristics of all trees.

Plato believed that knowledge derived from the senses is deceptive and that true knowledge comes from understanding these ideals. This perspective fundamentally shaped Western philosophy and emphasized the difference between the material and the immaterial. The other philosophers listed each had significant contributions to philosophy as well, but none are primarily associated with an idea system that parallels the depth and complexity of Plato's Theory of Ideas.