The principle of non-contradiction (PNC) is both a law of being and a law of thought. Based on the text of Aristotle, the aim of this book is to open a new discussion around the possibility of knowing the PNC by way of intuitive understanding, by recognising its indemonstrability and unhypothetical character and through its fundamentally being the basis of all demonstrative sciences.Through analysis of contradictory terms and confutational proof, it is possible to find in Aristotle's Metaphysics, book IV the fundamental points by which the PNC emanates from being and protects the unity of thought from contradiction.The PNC emanates from being because it defines it, and protects the unity of thought because the non-contradictory nature of being is the content of one's thought. Therefore, to deny it, does not mean that reality itself is contradictory by nature, but merely shows lack of education on the part of the person who denies the principle. In fact, the PNC is the highest criterion of significance in its ontological formulation.Introduction - Chapter I: The PNC as a law of reality and thought - 1. The PNC as a law of reality - 1.1 'The same attribute cannot belong and not belong' - 1.2 Relatives and contradictories - 1.3 Contraries and contradictories - 1.4 Privation and possession and contradictories - 1.5 Contradictory propositions and contradictory terms - 1.6 'The same subject' - 1.7 Being as an analogous term - 1.8 Substance as the strongest meaning of being - 1.9 'The same respect' - 1.10 'At the same time' - 2. The PNC as a law of thought - 2. 1 The act of believing - 2. 2 Contradictory beliefs - 2.3 The impossibility to believe contradictory as a unity of thought - 2.4 Apparent contradiction - Chapter II: The PNC as indemonstrable first principle - 1. 'The one who is responsible for knowingthe PNC is a philosopher' - 2. 'An unhypothetical principle' - 3. 'The PNC as the basis of all demonstration' - 4. The indemonstrability of PNC and the possibility to demonstrate it by way of dialectical argument - 1 The indemonstrability of the PNC - 4.2 Proper demonstration and the dialectical argument - 5. Confutational proof - Conclusion - Bibliography - 1. Primary Sources - 2. Selected Bibliography - Index of Names