Theory of Forms
Plato held rational insight to a very high standard, as is seen in his works such as Meno and The Republic. He taught on the Theory of Forms (or the Theory of Ideas) which asserts that the highest and most fundamental kind of reality is not the material world of change known to us through sensation, but rather the abstract, non-material (but substantial) world of forms (or ideas). For Plato, these forms were accessible only to reason and not to sense. In fact, it is said that Plato admired reason, especially in geometry, so highly that he had the phrase "Let no one ignorant of geometry enter" inscribed over the door to his academy.