 In chapter I.5 of his Metaphysics, Avicenna argues that just as there are first and self-evident sentences, there are also fundamental, first known, self-evident concepts, which are common to all beings: “the existent”, “the thing”, “the necessary”. It was due to Avicenna that the primum cognitum, the first object of knowledge became a central topic of medieval Latin metaphysics. The question of the primum cognitum was variously answered. For Guibert of Tournai, Bonaventure and Henry of Ghent, God is the primum cognitum (Goris 1999), for Thomas Aquinas and Duns Scotus being (ens), for Berthold of Moosburg the good (bonum). An example of a primum cognitum theory influenced by Avicenna is Thomas Aquinas’. There cannot be an infinite regress, neither in the order of demonstration, nor in the order of definition, Thomas argues. This is why there is a first concept: “being”. It is what is first grasped by the mind, and it is unrestrictedly universal. It is specific for Thomas, however, that the order of definition and of demonstration are not on the same level. The principle of contradiction rests (fundatur) on a conceptual basis, since it can be reduced to the terms out of which it is composed, among which “being” is the first (In Metaph. IV.6; Aertsen 1996, 146–151).