I am not quite sure why
Unam Sanctam has been brought up; I was not referring to it in the post which Mr. Higgins cites. In any case, however, I am mystified as to why one might claim that it is not infallible as per the specifications of
Pastor Æternus. According to the council, the Pope speaks infallibly when “in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole Church,” as “he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his Church to enjoy in defining doctrine concerning faith or morals.” The definition at the end of
Unam Sanctam meets these standards, as evidenced by the formula “
declaramus, dicimus, definimus, et pronunciamus” which Pope Boniface employs.