Leo XIII on the restoration of Christian philosophy

By Thomas V. Mirus ( bio - articles - email ) | Jul 08, 2025 | In Magisterium of Leo XIII

The election of Pope Leo XIV has prompted a revival of interest in the teachings of his namesake, Pope Leo XIII, whom he has cited as an inspiration. Pope Leo XIII’s encyclicals Rerum Novarum and Immortale Dei, have significantly influenced my own thinking, and even before our present Leo’s election, I was thinking of making a wider survey of the Leonine magisterium. Now seems like the perfect moment to launch this series of articles on the most important encylicals of Leo XIII.

Leo XIII is, of course, best known for pioneering what has come to be known as Catholic social teaching. While the series begun with this article will cover more than just social encyclicals, my primary objective is to show the unity of Catholic social teaching beyond just “economics”. We often recall Leo’s “economic teachings” while leaving aside other teachings which became less fashionable after Vatican II, yet which provide the foundation for a proper understanding of “social teaching” more narrowly considered—such as the hierarchical structure of the family, the true meaning of liberty, and the proper relation between Church and state. Reading beyond Rerum Novarum (or even just reading that encyclical carefully), one quickly becomes aware of the close relation between these various spheres.

Pope Leo XIII was born in 1810, elected in 1878, and died in 1903. Starting his life as a public intellectual when he was nearly seventy, he wrote eighty-eight encyclicals. I will be following the selection of thirty-one letters in Cluny Media’s two-volume set, The Great Encyclicals of Pope Leo XIII—though I will not follow the collection’s chronological order or, in some cases, its division between Leo’s “social” and “spiritual” letters. (For example, the encyclical on marriage, Arcanum Divinae, is placed in the second volume of “spiritual letters”, despite being foundational to Leo’s social magisterium.) The encyclicals may also, of course, be read on the Vatican website, but I prefer reading print to digital.

This series will be more about the texts themselves than about their historical context, but I can recommend Russell Hittinger’s recent book On the Dignity of Society for some very useful essays on the context and historical significance of Leo’s social magisterium.

One more thing: each of these articles will be accompanied by a longer podcast discussion (embedded at the bottom of the page). Now, let’s seize the Leonine moment!

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Rather than jumping immediately into the social teachings, it seems appropriate to begin a survey of the Pope Leo XIII’s magisterium with the higher-level considerations introduced in his third encyclical, Aeterni Patris, on the restoration of Christian philosophy. This is the encyclical which famously called for a revival of the thought of St. Thomas Aquinas, and the year of its publication (1879) also saw Leo establish the Pontifical University of St. Thomas Aquinas—with his brother, Giuseppe Pecci, as its first rector.

It seems that since at least the 18^th century, Catholic engagement in philosophy had been something of a mess. Catholic philosophers would take bits and pieces from modern philosophers like Descartes, Kant, and Hegel, attempting to turn them to some Catholic purpose, but this produced nothing very cohesive or robust, with negative consequences for Catholic intellectual culture and education. In this context, Leo called for a return to the great Scholastic tradition, particularly the teaching of the Angelic Doctor.

Here are some major takeaways from Aeterni Patris:

Grace and revelation are all-important, but the light of reason is a gift from God which we ought to use. While some truths may only be known by revelation, others may be discovered by reason alone, such as the existence of God, His infinite perfection, His goodness and trustworthiness. These truths known by philosophy prepare the way for faith. Reason can discover signs of the credibility of the Gospel and the divine institution of the Church. While philosophy can be misused to attack the Church, it also enables believers to defend the faith using their enemies’ own weapons.

From the very beginning, Catholics made use of reason as a support for faith, and philosophy (including the insights of pagan thinkers) as an aid to theology. Leo shows this by surveying the Church’s early history from St. Paul, to St. Justin Martyr and the early apologists, to St. Augustine. (Leo says Augustine “would seem to have wrested the palm” from all other Church Fathers, at least when it comes to the use of philosophy to serve theology.) Later, the Scholastics, the leading lights of whom were St. Thomas Aquinas and St. Bonaventure, collected and synthesized the wisdom of the Fathers while continuing to use philosophy in the service of systematic theology.

Philosophy is necessary for the development of theology as a science. Scientific theology, with the aid of philosophy, sets the revealed doctrines in their proper order, elucidates their principles and their interconnections, and reveals the organic unity of the whole. It also learns about supernatural things by analogy from things that are naturally known. Philosophy thus has more than an apologetic or preparatory function; it deepens our understanding of the revealed truths themselves.

Faith and reason are fully compatible. The Church has always understood that truth cannot contradict truth. So far from fearing that the authority of doctrine will stifle their intellectual endeavors, philosophers and other scientists should trust that the rule of faith protects them from falling into error. But this requires humility by which philosophy lets itself be judged, rather than judging the faith or trying to surpass it. Philosophy must “esteem it the highest honor to be allowed to wait upon heavenly doctrines like a handmaid and attendant”. Leo singles out the historical tradition that best exemplifies this harmonious relationship: “it is the proper and special office of the Scholastic theologians to bind together by the fastest chain human and divine science.”

St. Thomas Aquinas is the Catholic philosopher par excellence. Citing many Popes, councils, and religious orders that had already honored St. Thomas, Pope Leo XIII confirms his place as the preeminent teacher of theology for the whole Church.

Above all, it is St. Thomas’s principles which are important:

...the Angelic Doctor pushed his philosophic inquiry into the reasons and principles of things, which because they are most comprehensive and contain in their bosom, so to say, the seeds of almost infinite truths, were to be unfolded in good time by later masters and with a goodly yield. And as he also used this philosophic method in the refutation of error, he won this title to distinction for himself: that, single-handed, he victoriously combated the errors of former times, and supplied invincible arms to put those to rout which might in after-times spring up. Again, clearly distinguishing, as is fitting, reason from faith, while happily associating the one with the other, he both preserved the rights and had regard for the dignity of each; so much so, indeed, that reason, borne on the wings of Thomas to its human height, can scarcely rise higher, while faith could scarcely expect more or stronger aids from reason than those which she has already obtained through Thomas.

This is why the Thomistic revival is so crucial for Leo—because his principles are endlessly fecund and can be applied in fresh ways to new problems in theology, in science, and in social life. This was amply borne out in the social teaching of Leo and numerous subsequent Popes, as well as the many great Thomists who later arose in response to Leo’s call.

The importance of St. Thomas to a correct doctrine of civil society. Russell Hittinger points out that when Leo references St. Thomas throughout his magisterium, it is usually in a socio-political context—his great concern is to use St. Thomas to develop a civil doctrine in response to the political crises of the 19^th century. Leo cites Thomas’s teachings “on the true meaning of liberty, which at this time is running into license, on the divine origin of all authority, on laws and their force, on the paternal and just rule of princes, on obedience to the higher powers, on mutual charity one toward another”.

But this was not to lay out a single, rigid interpretation of how St. Thomas’s thought might be applied to political theory, much less present-day political conditions. Popes and Catholic thinkers throughout the next century developed varying applications of Thomism to the political realm and, just as Leo did, “engaged creatively” (as Hittinger puts it) with modern regimes.

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What struck me in reading Aeterni Patris is, surprisingly, how little of it was new to me. Catholics today take the cohesion of faith and reason for granted—anyone with a decent Catholic high school education, or who has watched a few Bishop Barron videos, knows that the Church considers philosophy the handmaid of theology. But leading up to Leo’s time, this value of philosophy to the Church had become somewhat obscured. It is precisely because of Leo’s initiative that we have benefited from three or four waves of a Thomistic revival in the intervening 150 years.

It should be remembered, too, that Leo called for a return to the Scholastic tradition in general (though not indiscriminately), with St. Thomas as its greatest but not only light. Though St. Thomas remains the common doctor of the Church, Pope Leo XIII would no doubt be pleased to see the ongoing revival of interest in St. Bonaventure and the Franciscan Scholastics.

Finally, Leo asks us always to remain humble in our studies, in imitation of St. Thomas, “who never gave himself to reading or writing without first begging the blessing of God, who modestly confessed that whatever he knew he had acquired not so much by his own study and labor as by the divine gift”.

Listen to me discuss Aeterni Patris at length on the Catholic Culture Podcast: