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Plato´s theory of ideas prevailed over 2.000 years and lost influence during the 17th century. It is a theory of the world and of mind likewise. It will be shown that Piagetian psychology has the tools available to illuminate the very foundations of the theory of ideas. The confusion of subjective and objective typical to the preoperational stage is the key to understand the theory of ideas. Phenomena discovered by Piagetian psychology to describe the mental world of the preoperational stage such as eidetism, non-differentiation between perception and representation, non-conservation of physical entities, conceptual realism, law of participation, animism, artificialism, and belief in metamorphosis are necessary to understand the theory of ideas. The developmental approach goes much beyond the possibilities of interpretation put forward by traditional historians of philosophy. Piaget himself had done a lot to interpret the history of sciences and philosophy in terms of psychological stages but did not devote a complete work to the theory of ideas.