31 These emphasize the state and condition of Christ's human nature to such an extent as to make it seem something existing in its own right ( subjectum quoddam sui juris ), and not as subsisting in the Word itself. But the council of Chalcedon in full accord with that of Ephesus, clearly asserts that both natures are united in 'One Person and subsistence', and rules out the placing of two individuals in Christ, as if some one man, completely autonomous in himself, had been taken up and placed by the side of the Word. St. Leo not only adheres to this opinion (i.e. that of Chalcedon), but he also indicates the source whence he derives his sound doctrine. 'Whatever', he says, 'we have written has manifestly clearly been taken from the doctrine of the Apostles and of the Gospels' ( Ep . clii. PL . liv, 1123).
Source: Sempiternus Rex Christus (Vatican.va)