That ultimate responsibility for true Christian doctrine is included in the universal primacy which the bishop of Rome as successor of St Peter possesses in the universal Church is a claim that should surprise no one who is prepared to entertain the notion of a universal primacy in the first place. This is, at any rate, the burden of the famous chapter 4 of “Pastor Aeternus”, the document in which Vatican I defined the infallible teaching of the bishop of Rome. What, with the approval of the Council, Pope Pius IX defined in the text that was promulgated on 18th July, 1870, as “dogma divinely revealed” runs as follows:

“The bishop of Rome, when he speaks ex cathedra, i.e. when, in discharge of the office of pastor and teacher of all Christians by virtue of his supreme apostolic authority, he defines a doctrine regarding faith or morals to be held by the universal Church, is, by God’s, help promised to him in St Peter, possessed of that infallibility with which the divine Redeemer willed that his Church should be endowed in defining a doctrine regarding faith or morals; and such definitions of the bishop of Rome are therefore of themselves, and not from the consent of the Church, irreformable”.

Assuming, then, that the bishop of Rome as successor of St Peter must in some sense be “the mouth of the Church and of the episcopate”, and must thus be able on some occasions at least to give voice to the faith that all alike hold, what are the problems of interpretation that this text contains? The fact that these problems have often been raised before, and that, in trying to resolve them, we have little new to say, will not deter us from covering the ground once again.