First, and most simply, he told the truth – that simple and unfashionable purpose of philosophy that is so often fudged (“nuanced”) or forgotten today. The following quotation should be chiseled on the doorposts of every philosophy department in the world: “The study of philosophy is not the study of what men have opined, but of what is the truth.”
[Secondly,] Descartes says that the one thing he learned about philosophy in the university that stuck with him was that one could not imagine any doctrine so bizarre or unbelievable that it has not been seriously taught by some philosopher or other. What was true already in 1637 is triply true today. St. Thomas, however, is the master of common sense. He has an uncanny knack of sniffing out the obviously right position amid a hundred wrong ones. This holds true especially in ethics, the real test of a philosopher. Some great philosophers, like Descartes, Hegel, and Heidegger, have no philosophical ethics at all. Others, like Hobbes and Hume and Kant and Nietzsche, have ethics that are simply unlivable. St. Thomas is as practical and plain and reasonable in ethics as Aristotle, or Confucius, or your uncle.
[Thirdly,] St. Thomas was a master of metaphysics and technical terminology, yet he was also such a practical man that, as he lay dying, he was talking about three things: a commentary on “The Song of Songs,” a treatise on aqueducts, and a dish of herring. Ordinary people, popes, and kings wrote to him for advice and always got back sound wisdom. I know of no one since St. Paul who is so full of both theoretical and practical wisdom.
[Fourthly,] Those who love truth passionately usually also love simplicity and clarity of style so that as many people as possible can benefit from this precious thing – truth. Fr. Norris Clarke, SJ, of Fordham University, the most Aquinas-like mind I know of all men living, says there are three kinds of philosophers: those who, at first, seem clear but, upon further readings, become more and more obscure; those who, at first, may seem obscure but become clearer and clearer upon each reading (St. Thomas is the prime example of this kind); and those who seem obscure at first and remain obscure.
St. Thomas aimed only for light, not heat. There is almost never anything personal in the “Summa,” no rhetoric, no appeal to the irrational – nothing but lucidity.
[Fifthly,] And depth – no philosopher since St. Thomas has ever so successfully combined the two fundamental ideals of philosophical writing: clarity and profundity. Continental European philosophy, in this century, has sought and, sometimes, found depth by focusing on the truly fundamental issues, but at the expense of clarity. English philosophy has sought and, often, found clarity, but at the expense of depth, concentrating on second-order linguistic questions rather than on those the average person wonders at: God, man, life, death, good, and evil.
From: A Summa of the “Summa”: The Essential Philosophical Passages of St. Thomas Aquinas’ “Summa Theologica” Edited and Explained for Beginners by Peter Kreeft (San Francisco: Ignatius Press, 1990), pp. 11-12.