In many of his writings, Thomas Aquinas discussed the role of human reason, a central issue for all Scholastic philosophers. In these brief selections from two of his long works, he examines the development of reason through time and its natural distribution among people, and explores the relationship between reason and faith.

From Summa Theologica

Human reason naturally moves step by step from the imperfect to the perfect. Whence we see in the speculative sciences that those who first philosophized provided imperfect results which were afterwards made more perfect by their successors. Similarly in practical matters, those who first tried to find something useful for the human community, being unable to understand everything, introduced things in many ways deficient; but their successors replaced these with ones far less faulty….

It may be said that a good proportionate to the ordinary state of nature is found in many and is lacking in few, but that a good that exceeds that ordinary state is discovered only in very few. It is evident that there are many who have knowledge sufficient for governing their own lives, and few, who are called fools or idiots, who lack this knowledge, but there are also very few who are able to attain a profound knowledge of intelligible things [that is, of philosophy]. Since, therefore, eternal blessedness, which consists in the vision of God, exceeds the ordinary state of nature (especially because grace has been weakened by the corruption of original sin), there are very few indeed who are saved.

Commentary on the book On the Trinity

[Philosophy] first proves things one needs to know in order to understand the faith … things proved by natural reason concerning God, namely, that He exists, that He is one, or similar things concerning Him or his creatures. Second, to make known things of the faith by means of analogy, just as Augustine in his book On the Trinity used many analogies taken from philosophy in order to explain the Trinity….

The gifts of grace are added to nature so that they do not do away with it, but instead perfect it. Hence the light of faith freely infused into us does not destroy the light of natural knowledge [reason] implanted in us naturally. Although the … human mind cannot show us things made manifest by faith, it is nonetheless impossible that that which faith gives is contrary to that implanted in us by nature. Were that the case, one or the other would be false, and, since God gave us both, He would be the author of untruth, which is impossible…. Just as sacred doctrine is founded on the light of faith, so is philosophy founded on the light of natural reason, and hence it is impossible that philosophical things are contrary to the things of faith.

Source: Quoted in J. H. Mundy, Europe in the High Middle Ages, 1150–1300, 3d ed. (Harlow, U.K.: Pearson-Longman, 2000), pp. 296–297, 305, 312, 325. Copyright © Pearson Education Limited, 2000. Used by permission of Pearson Education.

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