This work throws a new light on Xenocrates, the third leader of the Academy, who is considered in the existing research as merely a derivative doctrinaire. This complete presentation of his philosophy encompasses, in addition to cosmology, theology and ontology, for the first time also his doctrine of principles. This is found in the renowned report of Sextus, Adv. Math., X, §§ 248-283, which can be read anew against the background of the philosophy of Xenocrates. The doctrine of monistic principles proffered in the passage from Sextus derives from Xenocrates, who inserts his own precepts in his report on Platos unwritten teachings. Xenocrates' doctrine of principles constitutes the link between the ontological metaphysics of Aristotle and Plato's metaphysics of unity; Xenocrates' absolute monas is both in its essence connected with the nous, and ontologically transcendent. Xenocrates proves himself to be a thinker who is true to Plato and innovative. He is not only the precursor to middle-Platonism, but was also considered amongst the students of the inner circle of the Academy to be the 'rock' upon which Plato was able to establish his teachings and his school.