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An encyclical letter of Pope Leo XIII (issued 4 August, 1879); not to be confused with the apostolic letter of the same name written by Pope Pius IX.

Its purpose was the revival of Scholastic philosophy, according to the mind of St. Thomas Aquinas. It opens with the consideration that the Church, although officially the teacher of revealed truth only, has always been interested in the cultivation of every branch of human knowledge, especially of philosophy on which the right cultivation of other sciences in great measure depends. But the Pope declares that the actual condition of thought makes it a duty for him to do something for the study of true philosophy ; because many present evils are to be ascribed to false philosophy, inasmuch as, since man is naturally led by reason, whither the reason leads the will easily follows. The Encyclical then shows how rational philosophy prepares the motives of credibility in matters of faith, and explains and vindicates revealed truths. But the truth unfolded by reason cannot contradict the truths revealed by God ; hence, although in the pursuit of natural know ]edge philosophy may justly use its own method, principles, and arguments, yet not so as to withdraw from the authority of Divine revelation . The Encyclical next shows, by extracts from many Fathers of the Church , what reason helped by revelation can do for the progress of human knowledge. Then came the Scholastics of the Middle Ages, who brought together and bound into one harmonious whole, by a system of philosophy, the Christian wisdom of the Fathers. Since it was the work of the Scholastic theologians, according to the Encyclical, to unite divine and human science, their theology could never have succeeded, as it did succeed, if their philosophy had not been a complete system.

Leo XIII then marks out St. Thomas as the prince of the Scholastic theologians and philosophers, for which he finds evidence in the acknowledgment of the universities, of popes, general councils, and even of those outside the Church, one of whom boasted that if the works of St. Thomas were taken away he would fight and defeat the Church. That accounts for the unrelenting war which has been made against Scholastic philosophy since the Reformation arose. The Encyclical points out how some have turned away from it, but passes on to show how it can help in the pursuit of metaphysical and social science. It also insists that St. Thomas constantly founded his reasons and arguments on experiments; in the course of the centuries which have passed since his time, experiments have, of course, been disclosing facts and secrets of nature ; nevertheless the writings of St. Thomas bear witness that the experimental spirit was as strong in him as it is in us. Hence, in the Pope's appeal to the bishops of the Christian world to, help in restoring and spreading the "wisdom" ( sapientiam ) of St. Thomas, he repeats, Sapientiam Sancti Thomae dicimus , because, as he explains, he does not at all ask to have the excessive subtleties of some scholastics revived, nor opinions which later investigations have exploded. 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. It is worthy of remark that Leo XIII, following up the Encyclical, addressed (15 October, 1879) a letter to Cardinal de Luca in which, besides ordering that the philosophy of St. Thomas be taught in all the Roman schools, he founded the "Accademia di San Tommaso", and made provision for a new edition of St. Thomas's works. The Accademia has done much to help on the movement thus inaugurated, and a Collegium of Dominican Fathers have ever since been working at the new (Leonine) edition of St. Thomas. A great part of the work has already been done, but all will not be completed for some years to come.

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