The contribution to christology made by Leontius of Byzantium (fl. 520–543 A.D.) lies in his doctrine that the manhood of Christ is ‘enhypostatic’ (ἐνυπόστατος)—the doctrine of Enhypostasia or Inexistence. The Council of Chalcedon (451) defined the person or hypostasis of Christ as consisting in the union of two perfect natures, the nature of God and the nature of man, without division or confusion. On the principle laid down by Aristotle and accepted by all disputants, that there can be no such thing as a nature or substance without hypostasis (ϕύσις, οὐσία ἀνυπόστατος), the question arises of how it possible to avoid the conclusion of two hypostases, corresponding to the two perfect natures, in Christ. Such was the problem set by the definition of Chalcedon and the logic of Aristotle.