Certain truths which, though not revealed by God, come nevertheless under the teaching authority of the Church because of their close connection with revealed doctrines.
They are involved, for instance, in such questions as these: Was the election of Pius XI canonical, so that he is the rightful successor of Saint Peter?
Are the Saints canonized by the Church really in heaven?
Is this or that condemned teaching really contained in a certain book?
If the Church did not enjoy infallible authority to determine such matters, it would be practically impossible for her to carry out her Divine mission.
New Catholic Dictionary