イデア論
Mathematical Platonism is the form of realism that suggests that mathematical entities are abstract, have no spatiotemporal or causal properties, and are eternal and unchanging. This is often claimed to be the view most people have of numbers. The term "Platonism" is used because such a view is seen to parallel Plato's Theory of Forms and a "World of Ideas" (Greek: "eidos" (εἶδος)) described in Plato's allegory of the cave: the everyday world can only imperfectly approximate an unchanging, ultimate reality. Both Plato's cave and Platonism have meaningful, not just superficial connections, because Plato's ideas were preceded and probably influenced by the hugely popular "Pythagoreans" of ancient Greece, who believed that the world was, quite literally, generated by numbers.