[1] It was subtitled "On the Restoration of Christian Philosophy in Catholic Schools in the Spirit (ad mentem) of the Angelic Doctor, St. Thomas Aquinas".

In August 1879, eighteen months into his pontificate, Pope Leo XIII (formerly Joachim Cardinal Pecci, bishop of Perugia), issued the encyclical letter Aeterni Patris.

[3] The purpose of Leo XIII was the revival of St. Thomas's philosophy and the continuing of his spirit of investigation, but not necessarily the adoption of every argument and opinion to be found in the works of the scholastics.

The encyclical, however, was no surprise to any acquainted with Cardinal Pecci, who had for years been spearheading a Thomistic renaissance in the schools in his diocese of Perugia, leading to such theologians and philosophers as Reginald Garrigou-Lagrange, Etienne Gilson, and Jacques Maritain.

"Zigliara also helped prepare the great encyclicals Aeterni Patris and Rerum novarum and strongly opposed traditionalism and ontologism in favor of the moderate realism of Aquinas.

The opening paragraph begins with a reference to Christ’s command to His Apostles to set all men free by teaching the truth of the faith to all nations (Matthew 28:19).

This, then, is the aim of Aeterni Patris: to promote the kind of philosophy that “shall respond most fitly to the excellence of faith, and at the same time [is] consonant with the dignity of human science.” 2.

Moreover, Philosophy complements theology in that true understanding and knowledge are better and more easily attained by those who join together philosophic studies with the love of faith and integrity of life.

The Church “commands” Christian teachers to enlist the help of philosophy in instructing the faithful but cautions against philosophical arguments that are contrary to revealed truth.

“Faith frees and saves reason from error, and endows it with manifold knowledge.” Revelation serves as a lamp for the philosopher’s path toward truth.

He turns his attention first to St. Augustine, who Leo says “[w]ould seem to have wrested the palm from all.” The genius of Augustine was his ability to combat “most vigorously all the errors of his age,” as well as his ability to lay down the “safe foundations and sure structure of human science.” The paragraph ends with mention of John Damascene, Basil, and Gregory of Nazianzen as carrying the tradition in the East, and Boethius and Anselm in the West, all of whom Pope Leo says “[a]dded largely to the patrimony of philosophy.” 14.

Paragraph 14 speaks of the “doctors of the middle ages,” the “Scholastics,” whose contribution to the tradition was their “diligently collecting, and sifting, and storing up, as it were, in one place, for the use and convenience of posterity the rich and fertile harvests of Christian learning scattered abroad in the voluminous works of the holy Fathers.” Pope Leo XIII quotes Pope Sixtus V, who names in particular the “angelic St. Thomas and the seraphic St. Bonaventure” as preeminent doctors whose “surpassing genius” and “unwearied diligence” aided in further advancing the tradition.

The encyclical describes how many religious orders mandated the study of his works to their members: the Dominicans, of course, but also the Benedictines, Carmelites, Augustinians, and Jesuits.

The twentieth paragraph of the encyclical refers to all the great universities of the scholastic period where Thomistic theology enjoyed prominence, creating harmony among all the schools adhering to his teaching.

Thomas Aquinas has long been held “in singular honor” in the ecumenical councils, including those of Lyons, Vienne, Florence, and the Vatican.

Their inventions were in accordance with their own whims, and so, naturally, there arose a great diversity of philosophies, “differing and clashing” even in those matters of human knowledge that are most important.

Scholastic philosophy, with its “sound judgment,” “right method,” and “spirit,” will strengthen and guide all the different disciplines, including the liberal arts and the physical sciences.

This paragraph emphasizes that the revival of scholasticism and, in particular, Thomism for which the encyclical calls is not a recommendation to practice a slavish adherence to outdated doctrines and ideas: “if there is anything that ill agrees with the discoveries of a later age … it does not enter Our mind to propose that for imitation in Our age.” It is crucial, as well, that Aquinas’s own writings be read and studied, rather than derivative works that may already be affected by errors.

The chief aim of Aeterni Patris was the reintroduction of Thomism into Catholic educational centers for the purpose of bringing faith and reason back into a fruitful dialectic.

French Dominicans Pierre Mandonnet (1858–1936), Marie-Dominique Chenu (1895–1990), and Yves Congar (1904–1995) explored the historical background of Thomas Aquinas and its relationship to theology.

One of the best known French Dominicans of the twentieth century was Réginald Garrigou-Lagrange (1877–1964), who influenced generations of students during his tenure at the Pontifical University of St. Thomas Aquinas (Angelicum) in Rome.

The spirit and thought of Saint Thomas Aquinas advocated by the encyclical has proven a valuable resource for Catholic philosophy and theology in bringing both faith and reason to bear on the problems of modern life.

Each was given defense from a canonical piece published in the late 19th century (the 9th Edition of the Encyclopædia Britannica, Nietzsche's Genealogy of Morals and Pope Leo XIII's Aeterni Patris).

MacIntyre ultimately conducts a complex series of both interior and exterior critiques of the encyclopaedic and genealogical positions in an attempt to vindicate philosophical Thomism as the most persuasive form of moral inquiry.