In Metaph. Β and Γ, Aristotle claims that universal principles, such as the principle of non contradiction or the principle of excluded middle, are principles of demons­trations. Besides the technical question of the way in which such principles are principles of demonstrations, this paper raises the question why Aristotle insists on their relation to demons­trations in the Metaphysics and not in Analytics. The answer proposed is that demons­trations use these universal principles because they are to reveal the stucture of reality, (...) and that these principles are principles of reality.But in calling these principles « syllogistical » principles too, Aristotle seems to assign to them a quite different role which has not been theorized. On one hand, this determination is inconsistent with the two other ones (laws of being, principles of demonstrations) since a law of reasoning as such belongs to formal logic and not to ontology. But on the other hand, Aristotle gives himself (Prior Analytics, I, 15) examples of valid syllogisms which admit of contradiction in the premisses, which means that he was perfectly aware of this difficulty, even though he left indeterminate the logical role of these universal principles. (shrink)