One of the most controversial texts in the history of philosophy is Aristotle's Metaphysics Lambda, which deals with the existence and attributes of the First Principle. One of the important controversies in the interpretation of this book is Aristotle's understanding of the causality of the First Principle: is the First Principle the agent of motion or the end of motion? This essay attempts to show, in light of Avicenna's brief interpretation of Lambda, that Aristotle's First Principle cannot be understood simply in the common dualistic framework of "final /efficient causality" of motion; rather, the internal structure of chapters 6 to 10 of Lambda, if reread with a consistent approach and in light of Avicenna's interpretative references, introduces the First Principle not only as the efficient/ultimate cause of motion, but also as "pure actuality" as a condition for the possibility of the consistency and coherence of the entire universe. Accordingly, in Lambda 6, the priority of actuality over potentiality is understood in the horizon of ontological priority; in Lambda 7, the expression “because of love” refers to the mode of effect, not the ultimate delimitation of God’s existential order; in Lambda 9, the pure actuality of the First Principle is explicitly established; and in Lambda 10, God appears as the unifying principle and the comprehensive good of the universe. Hence, in his interpretation of Lambda, Avicenna, by creating a methodological break from physics to ontology, elevates the causality of the First Principle to necessity and ontological dependence.