Jan. 28 is the feast day of St. Thomas Aquinas.

It offers us a gentle inducement and a great opportunity to reflect on the prodigious contribution of the most prolific and esteemed of all the doctors of the Catholic Church.

I am currently teaching a course that introduces the thought of this philosophical and theological giant to distance learning students. The main problem for my students is how to enter into the vast, organic system of thought that Aquinas has carefully knit together. Precisely because his thought is as thoroughly comprehensive and integrated as it is, it becomes a challenge for anyone to find a practical point of entrance.

This problem is particularly poignant since Aquinas’ thought is essentially grounded in and sustained by common sense. And a thinker who does not stray from common sense is vitally needed, for in the modern world, as G. K. Chesterton has pointed out, “nobody’s system of philosophy has really corresponded to everybody’s sense of reality.”

My choice for the elusive point of entrance is a fundamental declaration of realism that Aquinas offers in his monumental Summa Theologiae (I-II 93, 1 ad 3): “The human intellect is measured by things so that man’s thought is not true on its own account but is called true in virtue of its conformity with things.”

This statement, simple and direct, goes to the very heart of Aquinas’ common-sense realism. It reminds us of something that we already know, namely that what we know is not something that is on our heads, but something that exists outside the mind in the extra-mental world. It indicates that we are all facing the same reality, and suggests that as we come to know that reality better, we greatly improve our chances of sharing a common vision and consequently of living together in harmony. What we know is the basis for how we live. In Thomistic parlance, a sound epistemology provides the platform for a sound politics.

The simple observation of a child at play offers us sufficient corroboration of Aquinas’ thinking on this fundamental point. A child will manipulate his building blocks, let us say, until he discovers how they go together. Through a sometimes long and drawn-out process of trial and error, investigation and adjustment, he comes to learn how his blocks can be assembled. He is involved in what is called an “epistemic cycle” of experimenting with his playthings and learning about their objective constitution.

He is amused as he discovers their principles of operation. Playing is a method of learning.

But the primary principle that is uncovered here is that what one learns through play is about reality. Through a studious and persistent involvement with playthings, one learns about the objective order of things, and not about something that is already in one’s head. One learns by using one’s mind, but the content of what is learned lies outside the mind.

Aquinas possessed two virtues that are not often found in the same person to a high degree. Because he knew, convincingly, that what he sought existed outside of himself, he had a great deal of intellectual modesty. Secondly, because he allowed himself to be thoroughly convinced of what he did come to know, he possessed a goodly amount of intellectual audacity.

Today it is quite common to find thinkers who possess modesty without audacity. This unhappy combination, however, results in an appalling intellectual timidity. On the other hand, there are abundant thinkers who display great audacity, but without its justification through a prior modesty. In this case, the effect is a shallow intellectual arrogance.

Aquinas avoids the pitfalls of timidity and arrogance by deftly combining an intellectual modesty that is grounded in his awareness that, as Chesterton put it, “he knows nothing until he knows nothing,” and the audacity, linked to courage, not to turn his back on what he does come to know.

There are countless university students today who spend thousands of dollars to lose sight of the common-sense realism they once practiced when they were untutored children. They then proclaim that there is no basis for truth that is outside of the self. The subjectivism, relativism, skepticism and cynicism they espouse, however, are all enemies of common-sense realism and do not serve such students in any practical sense.

Adam and Eve were tempted to eat the forbidden fruit because they believed it to be a way of coming to “know good and evil.” Catholic scholars have explained the meaning of this curious phrase by relating it to the capacity to determine what is good and what is evil solely by oneself. This isolated and unrealistic way of determining things inevitably leads to ruin. It also sets man against God, and is the reason for the Fall.

Here we come to a correlative of Aquinas’ statement, cited above, indicating clearly that he is a theologian as well as a philosopher: “The divine intellect, on the other hand, is the measure of things, since things are true to the extent in which they represent the divine intellect.”

The entire thought of Aquinas is built upon the primary intuition that God is the Creator of a world of truths, and we are seekers and discoverers. To think that we are the creators of truth is, therefore, an attempt to self-divinize, and one that results in the unhappy rejection of God as well as self.

Donald DeMarco is adjunct professor

at Holy Apostles College

 and Seminary in Cromwell, Connecticut.