understand theology must familiarize himself with Scripture, the teachings of the fathers, and the decisions of councils, in such a way as to be able to make part of himself, as it were, those channels along which this divine knowledge flowed. Aquinas’s conception of reason is in some way parallel with his conception of revelation. Reason is in his idea not the individual reason, but the fountain of natural truth, whose chief channels are the various systems of heathen philosophy, and more especially the thoughts of Plato and the methods of Aristotle. Reason and revelation are separate sources of knowledge; and man can put himself in possession of each, because he can bring himself into relation to the church on the one hand, and the system of philosophy, or more strictly Aristotle, on the other. The conception will be made clearer when it is remembered that Aquinas, taught by the mysterious author of the writings of the pseudo-Dionysius, who so marvellously influenced medieval writers, sometimes spoke of a natural revelation, or of reason as a source of truths in themselves mysterious, and was always accustomed to say that reason as well as revelation contained two kinds of knowledge. The first kind lay quite beyond the power of man to receive it, the second was within man’s reach. In reason, as in revelation, man can only attain to the lower kind of knowledge; there is a higher kind which we may not hope to reach.
But while reason and revelation are two distinct sources of truths, the truths are not contradictory; for in the last resort they rest on one absolute truth—they come from the one source of knowledge, God, the Absolute One. Hence arises the compatibility of philosophy and theology which was the fundamental axiom of scholasticism, and the possibility of a Summa Theologiae, which is a Summa Philosophiae as well. All the many writings of Thomas are preparatory to his great work the Summa Theologiae, and show us the progress of his mind training for this his life work. In the Summa Catholicae Fidei contra Gentiles he shows how a Christian theology is the sum and crown of all science. This work is in its design apologetic, and is meant to bring within the range of Christian thought all that is of value in Mahommedan science. He carefully establishes the necessity of revelation as a source of knowledge, not merely because it aids us in comprehending in a somewhat better way the truths already furnished by reason, as some of the Arabian philosophers and Maimonides had acknowledged, but because it is the absolute source of our knowledge of the mysteries of the Christian faith; and then he lays down the relations to be observed between reason and revelation, between philosophy and theology. This work, Contra Gentiles, may be taken as an elaborate exposition of the method of Aquinas. That method, however, implied a careful study and comprehension of the results which accrued to man from reason and revelation, and a thorough grasp of all that had been done by man in relation to those two sources of human knowledge; and so, in his preliminary writings, Thomas proceeds to master the two provinces. The results of revelation he found in the Holy Scriptures and in the writings of the fathers and the great theologians of the church; and his method was to proceed backwards. He began with Peter of Lombardy (who had reduced to theological order, in his famous book on the Sentences, the various authoritative statements of the church upon doctrine) in his In Quatuor Sententiarum P. Lombardi libros. Then came his deliverances upon undecided points in theology, in his XII. Quodlibeta Disputata, and his Quaestiones Disputatae. His Catena Aurea next appeared, which, under the form of a commentary on the Gospels, was really an exhaustive summary of the theological teaching of the greatest of the church fathers. This side of his preparation was finished by a close study of Scripture, the results of which are contained in his commentaries, In omnes Epistolas Divi Apostoli Expositio, his Super Isaiam et Jeremiam, and his In Psalmos. Turning now to the other side, we have evidence, not only from tradition but from his writings, that he was acquainted with Plato and the mystical Platonists; but he had the sagacity to perceive that Aristotle was the great representative of philosophy, and that his writings contained the best results and method which the natural reason had as yet attained to. Accordingly Aquinas prepared himself on this side by commentaries on Aristotle’s De Interpretatione, on his Posterior Analytics, on the Metaphysics, the Physics, the De Anima, and on Aristotle’s other psychological and physical writings, each commentary having for its aim to lay hold of the material and grasp the method contained and employed in each treatise. Fortified by this exhaustive preparation, Aquinas began his Summa Theologiae, which he intended to be the sum of all known learning, arranged according to the best method, and subordinate to the dictates of the church. Practically it came to be the theological dicta of the church, explained according to the philosophy of Aristotle and his Arabian commentators. The Summa is divided into three great parts, which shortly may be said to treat of God, Man and the God-Man. The first and the second parts are wholly the work of Aquinas, but of the third part only the first ninety quaestiones are his; the rest of it was finished in accordance with his designs. The first book, after a short introduction upon the nature of theology as understood by Aquinas, proceeds in 119 questions to discuss the nature, attributes and relations of God; and this is not done as in a modern work on theology, but the questions raised in the physics of Aristotle find a place alongside of the statements of Scripture, while all subjects in any way related to the central theme are brought into the discourse. The second part is divided into two, which are quoted as Prima Secundae and Secunda Secundae. This second part has often been described as ethic, but this is scarcely true. The subject is man, treated as Aristotle does, according to his τέλος, and so Aquinas discusses all the ethical, psychological and theological questions which arise; but any theological discussion upon man must be mainly ethical, and so a great proportion of the first part, and almost the whole of the second, has to do with ethical questions. In his ethical discussions (a full account of which is given under Ethics) Aquinas distinguishes theological from natural virtues and vices; the theological virtues are faith, hope and charity; the natural, justice, prudence and the like. The theological virtues are founded on faith, in opposition to the natural, which are founded on reason; and as faith with Aquinas is always belief in a proposition, not trust in a personal Saviour, conformably with his idea that revelation is a new knowledge rather than a new life, the relation of unbelief to virtue is very strictly and narrowly laid down and enforced. The third part of the Summa is also divided into two parts, but by accident rather than by design. Aquinas died ere he had finished his great work, and what has been added to complete the scheme is appended as a Supplementum Tertiae Partis. In this third part Aquinas discusses the person, office and work of Christ, and had begun to discuss the sacraments, when death put an end to his labours.
The purely philosophical theories of Aquinas are explained in the article Scholasticism. In connexion with the problem of universals, he held that the diversity of individuals depends on the quantitative division of matter (materia signata), and in this way he attracted the criticism of the Scotists, who pointed out that this very matter is individual and determinate, and, therefore, itself requires explanation. In general, Aquinas maintained in different senses the real existence of universals ante rem, in re and post rem.
The best modern edition of the works of Aquinas is that prepared at the expense of Leo XIII. (Rome, 1882–1903). The Abbé Migne published a very useful edition of the Summa Theologiae, in four 8vo vols., as an appendix to his Patrologiae Cursus Completus; English editions, J. Rickaby (London, 1872), J. M. Ashley (London, 1888). See Acta Sanct., vii. Martii; A. Touron, La Vie de St Thomas d’Aquin, avec un exposé de sa doctrine et de ses ouvrages (Paris, 1737); Karl Werner, Der Heilige Thomas von Aquino (1858); and R. B. Vaughan, St Thomas of Aquin, his Life and Labours (London, 1872): other lives by P. Cavanagh (London, 1890); E. Desmousseaux de Giuré (Paris, 1888); M. Didot (Louvain, 1894). For the philosophy of Aquinas, see Albert Stöckl, Geschichte der Philosophie des Mittelalters, ii.; B. Hauréau, De la philosophie scolastique, vol. ii.; J. Frohschammer, Die Philos. d. Th. von A. (Leipzig, 1889); K. Prantl, Geschichte d. Logik, vol. iii.; C. M. Schneider, Natur, Vernunft, Gott (Regensburg, 1883), Das Wissen Gottes nach d. Lehre des Th. v. A. (4 vols. Regensburg, 1884–1886), Die socialistische Staatsidee beleuchtet durch Th. v. A. (Paderborn, 1894); A. Harnack, Hist, of Dogma (trans. Wm.
