Document Type : Research Paper
Based on insightful study on the theory of "first principle" and Plato's theory of "Forms", Aristotle has proposed the theory of Ousia and used it as basis of his ontology. However, this theory has been subject to some precise criticisms. Discussing the formation process of this theory, the present paper elaborates on some of its conceptological problems based on Aristotle sayings. "Non-prediction on subject" and "non-existence in subject", as two main features of Ousia, is a kind of attribution to generals and accepting contraries while holding numerical unity of Ousia is not acceptable, because it is subordinate to differentiation between psychic and relative features of object and Aristotle has not elaborated on this differentiation.