“[Infallibility] does not belong to the Roman Pontiff inasmuch as he is a private person, nor even inasmuch as he is a private teacher, since, as such, he is equal with all other private teachers and, as Cajetan wisely noted, equal does not have power over equal, not such power as the Roman Pontiff exercises over the Church Universal. Hence we do not speak about personal infallibility, although we do defend the infallibility of the person of the Roman Pontiff, not as an individual person but as the person of the Roman Pontiff or a public person, that is, as head of the Church in his relation to the Church Universal. Indeed it should not be said that the Pontiff is infallible simply because of the authority of the papacy but rather inasmuch as he is certainly and undoubtedly subject to the direction of divine assistance. By the authority of the papacy, the Pontiff is always the supreme judge in matters of faith and morals, and the father and teacher of all Christians. But the divine assistance promised to him, by which he cannot err, he only enjoys as such when he really and actually exercises his duty as supreme judge and universal teacher of the Church in disputes about the Faith. Thus, the sentence ‘The Roman Pontiff is infallible’ should not be treated as false, since Christ promised that infallibility to the person of Peter and his successors, but it is incomplete since the Pope is only infallible when, by a solemn judgment, he defines a matter of faith and morals for the Church universal.”
-Bishop Vincent Gasser in his relatio at Vatican I, found in the book "The Gift of Infallibility."