During the prelude to the First Vatican Council, the idea of a heretical pope was used as the primary argument against the solemn definition of papal infallibility. The medieval canonists and conciliarists had allowed for the notion of papal heresy by making a strict distinction between the apostolic seat itself and the individual occupants of the throne of Peter. However, when we examine the text of Pastor Aeternus in light of the contents of the official Relatio, which was drawn up at the council to explain the meaning of this document, we find that the above distinction used by the conciliarists was formally proscribed with an anathema. This article will argue that in doing so, the Council Fathers definitively excluded the possibility of a heretical pope.