This article is dedicated to analysis of formation, functioning and evolution of the concept of “idea”, its status and purpose. The article shows that the Ancient Greek (from Plato to Aristotle) dichotomy in the interpretation of the term “idea” gave the impetus for the emergence of the apriorism school of philosophy. According to Plato, an extremely general idea has the inherently absolute existence, with the status of a demiurge of reality, while according to Aristotle, ideas are like numbers, expressing the intelligible and eternal essence of things. Philosophers of both Antiquity and the Middle Ages believed the essence of things and processes to be eternal and unchanging, treating ideas as constants, relations and properties in the hierarchy of the world order. In Modern times, the development of analytical geometry and infinitesimal calculus determined the transition from constants to variables, which lead to revision of the concept of “idea”. The one-sidedness of rationalist and empirical traditions in the course of such rethinking lead to Kant’s transcendent apriorism, where ideas were concepts of reason. This completed the transformation of the “idea”: from an apt term to a sustainable philosophical concept.