67 Now it is of great importance, as We have said, that priests should have a learning adequate to the requirements of the age. For the attainment of this, in addition to a solid classical education, there is required both instruction and training in scholastic philosophy "according to the method, and the mind and the principles of St. Thomas Aquinas" - ad Angelicl Doctoris rationem , doctrinam et principia . This Our Illustrious Predecessor, Leo XIII, has called the philosophia perennis . It is essential to the future priest. It will help him to a thorough understanding of dogma. It will effectively forearm him against modern errors of whatever sort. It will sharpen his mind to distinguish truth from falsehood. It will form him to habits of intellectual clearness, so necessary in any studies or problems of the future. It will give him a great superiority over others, whose mere erudition, perhaps, is wider but who lack philosophical training.
Source: Ad Catholici Sacerdotii (Vatican.va)