(from An Introduction to Philosophy)
The Scholastic tradition, which grew up from the eighth century onward in the Christian West, was for long ignorant of Aristotle's original works, with the exception of the Organon (the treatises on logic), which had been translated into Latin by Boethius (480-526). But it was acquainted with his thought, which had been transmitted and popularised at second hand and formed an integral part of the great philosophic synthesis of late antiquity, Platonic though it was in the main, on which the Fathers, especially St. Augustine, had drawn so largely in the service of the faith, In the Christian schools Aristotle's logic was taught in Boethius's translation. But it was not until the latter part of the twelfth century that the other writings of the Philosopher (physics, metaphysics, ethics) began to reach the Schoolmen, mainly, it would appear, as a result of the ardent polemic conducted at that date by the leaders of Christian thought against the philosophy of the Arabs, who possessed these books together with the neo-Platonic commentaries in a Syrian version translated later into Arabic, and appealed to their authority. At first the object of considerable suspicion on account of the source from which they had been received and the mistakes which the Arab commentators had foisted into them, all the works of Aristotle were soon translated into Latin, at first from the Arabic text, later from the original Greek.
Now took place the meeting of human wisdom and divine truth, of Aristotle and the Faith. All truth belongs of right to Christian thought, as the spoils of the Egyptians to the Hebrews. "Whatever has been well said anywhere belongs to us who are Christians" -- because according to that saying of St. Ambrose, which St. Thomas delighted to quote, every truth, whoever said it, comes from the Holy Spirit. But someone must actually take possession, someone must enroll in the royal service of Christ the marvellous intellect of Aristotle. This work, begun by Albert the Great (1193-1280), was continued and brought to a successful conclusion by St. Thomas Aquinas (1225-1274). Its accomplishment demanded a rare conjunction of favourable conditions -- the ripe culture of the age of St. Louis, the magnificent organisation of the Dominican order, the genius of St. Thomas. St. Thomas, whom the Church has proclaimed Doctor par excellence, Doctor Communis Ecclesiae, and whom she has enthroned as the universal teacher of her schools, was not content with transferring the entire philosophy of Aristotle to the domain of Christian thought, and making it the instrument of a unique theological synthesis; he raised it in the process to a far higher order, and, so to speak, transfigured it.
He purged it from every trace of error -- that is to say, in the philosophic order, for so far as the sciences of observation or phenomena are concerned, St. Thomas was no more able than Aristotle to escape the errors prevalent in his day, errors which do not in any way affect his philosophy itself. He welded it into a powerful and harmonious system; he explored its principles, cleared its conclusions, enlarged its horizon; and, if he rejected nothing, he added much, enriching it with the immense wealth of the Latin Christian tradition, restoring in their proper places many of Plato's doctrines, on certain fundamental points (for example, on the question of essence and existence), opening up entirely new perspectives, and thus giving proof of a philosophic genius as mighty as that of Aristotle himself. Finally, and this was his supreme achievement, when by his genius as a theologian he made use of Aristotle's philosophy as the instrument of the sacred science which is, so to speak, "an impress on our minds of God's own knowledge," he raised that philosophy above itself by submitting it to the illumination of a higher light, which invested its truth with a radiance more divine than human. Between Aristotle as viewed in himself and Aristotle viewed in the writings of St. Thomas is the difference which exists between a city seen by the flare of a torchlight procession and the same city bathed in the light of the morning sun.
For this reason, though St. Thomas is first and foremost a theologian, we may as appropriately, if not with greater propriety, call his philosophy Thomist rather than Aristotelian.
This philosophy of Aristotle and St. Thomas is in fact what a modern philosopher has termed the natural philosophy of the human mind, for it develops and brings to perfection what is most deeply and genuinely natural in our intellect alike in its elementary apprehensions and in its native tendency towards truth. It is also the evidential philosophy, based on the double evidence of the data perceived by our senses and our intellectual apprehension of first principles -- the philosophy of being, entirely supported by and modelled upon what is, and scrupulously respecting every demand of reality -- the philosophy of the intellect, which it trusts as the faculty which attains truth, and forms by a discipline which is an incomparable mental purification. And for this very reason it proves itself the universal philosophy in the sense that it does not reflect a nationality, class, group, temperament, or race, the ambition or melancholy of an individual or any practical need, but is the expression and product of reason, which is everywhere the same; and in this sense also, that it is capable of leading the finest intellects to the most sublime knowledge and the most difficult of attainment, yet without once betraying those vital convictions, instinctively acquired by every sane mind, which compose the domain, wide as humanity, of common sense. It can therefore claim to be abiding and permanent (philosophia perennis) in the sense that before Aristotle and St. Thomas had given it scientific formulation as a systematic philosophy, it existed from the dawn of humanity in germ and in the prephilosophic state, as an instinct of the understanding and a natural knowledge of the first principles of reason and ever since its foundation as a system has remained firm and progressive, a powerful and living tradition, while all other philosophies have been born and have died in turn. And, finally, it stands out as being, beyond comparison with any other, one; one because it alone bestows harmony and unity on human knowledge -- both metaphysical and scientific -- and one because in itself it realises a maximum of consistency in a maximum of complexity, and neglect of the least of its principles involves the most unexpected consequences, distorting our understanding of reality in many directions.
These are a few of the external signs which witness to its truth, even before we have studied it for ourselves and discovered by personal proof its intrinsic certitude and rational necessity.