The Father William Most Collection
Infallibility of Level Three Teachings
[Published electronically for use in classes taught by Fr. Most and for private theological study.]
In Humani generis (DS 3885), 1950, Pius XII said that if the Popes in their acta deliberately take a position on an item currently being debated in theology, it is removed from debate, and falls under Lk 10:16: "He who hears you hears me". That is infallible.
But LG §25 seeming to speak of the same thing specifies only "religious submission", which is less than the infallible level.
We comment: If a Pope intends to make anything definitive, that is infallible. No special form of words is needed. Thus in the si quis dixerit... AS form, sometimes we find only disciplinary matters. While in Vatican I, DS 3006, we read that the books of Scripture have God as their author. But this was in a capitulum, not in a canon. Yet Pius XII in Divino afflante Spiritu EB 538, said that text of Vatican I was a "solemn definition".
We conclude that all that is required is the intent to make an item definitive, plus writing in such a way as to make that intent clear. The conditions given by Pius XII do make that clear, namely removing a thing from debate, and bringing it under Lk 10:16. Hence such things really are infallible.