Our local American Chesterton Society group gathers monthly to read and discuss an essay, poem, or a chapter of one of Chesterton’s books, sharing some snacks and libations. We’ve read Father Brown mysteries, essays, poetry (“The Ballad of the White Horse”), “The Everlasting Man”, and are now reading “St. Thomas Aquinas”. Next up: “St. Francis of Assisi”.
Recently, we discussed Chapter IV of “St. Thomas Aquinas” in which Chesterton outlines his interpretation of St. Thomas’s arguments against the Manichees (“A Meditation on the Manichees”) and explains why he had to argue against them in spite of the fact that “St. Dominic and Simon de Montfort between them had already pretty well settled the Manichees”. (“St. Thomas Aquinas”, San Francisco: Ignatius Press, 2002; p. 95)
Chesterton begins by describing the famous story of St. Thomas Aquinas attending a banquet hosted by St. Louis of France: the one in which Aquinas suddenly struck the table and cried out, “And that will settle the Manichees!” St. Louis immediately instructs his secretaries to go and help his guest capture his idea in writing. Chesterton notes that this episode demonstrates St. Thomas Aquinas’ dedication to thinking about the dangers posed to faith in his own time. Then of course, Chesterton points out how relevant these things are to us today. Like the poor, he might say, the Manichees are always with us: 
But it is always in one way or another a notion that nature is evil; or that evil is at least rooted in nature. The essential point is that as evil has roots in nature, so it has rights in nature. Wrong has as much right to exist as right. As already stated this notion took many forms. Sometimes it was a dualism, which made evil an equal partner with good; so that neither could be called an usurper. More often it was a general idea that demons had made the material world, and if there were any good spirits, they were concerned only with the spiritual world.”(p. 99)
Creation and Fall
Then someone in our group went back to a passage that surprised me with a new insight:
Now nobody will begin to understand the Thomist philosophy, or indeed the Catholic philosophy, who does not realise that the primary and fundamental part of it is entirely the praise of Life, the praise of Being, the praise of God as the Creator of the World. Everything else follows a long way after that, being conditioned by various complications like the Fall or the vocation of heroes. The trouble occurs because the Catholic mind moves upon two planes; that of the Creation and that of the Fall.”(pp. 97-98)
What Chesterton is describing is what I’ve often heard of as the Catholic “both/and”: we both love the world and try to be detached from it; we love nature because God created it; we recognize that we are part of nature because God created us; we also know that we sin and struggle with concupiscence and that nature has powers we cannot control and sometimes even comprehend. 
We fight against the world, the flesh, and the Devil; God created all three and they were good; then Satan and his minions fell and then Adam and Eve fell. As Chesterton notes, “the world and the flesh have been twisted by a bad intention called the devil. But he cannot make things bad; they remain as on the first day of creation. The work of heaven alone was material; the making of a material world. The work of hell is entirely spiritual.” (p. 100) 
This great mystery, these two planes of thought, Creation and Fall, are fundamental to, for example, our Sacramental system. Each Sacrament has three aspects: the Form, the Matter, and the Intent. The Matter of each Sacrament matters: Water and Oil for Baptism; Bread and Wine for Communion; a man and a woman for Matrimony, etc. If we think Matter is intrinsically evil, we won’t accept the Sacraments at all— or the Incarnation, since God became flesh.
Catholic Doctrine and Sanity
Finally, Chesterton makes a claim that I have heard from several other great writers, including Blessed John Henry Newman and the Anglican mystery writer Dorothy L. Sayers, when commenting on how Manichaeism confuses the way people think about the world: “In truth, this vividly illuminates the provincial stupidity of those who object to what they call "creeds and dogmas." It was precisely the creed and dogma that saved the sanity of the world.” (p. 102) The Catholic “both/and” way of thinking on “two planes; that of the Creation and that of the Fall” also helps save us from the insanity of the world.
It can help us understand the application of the Church’s teachings in our lives from the most spiritual aspects of our prayer life to the most practical actions of our moral life. To borrow an example from Chesterton’s "Orthodoxy", this is why the Church has at once emphasized celibacy and emphasized the family; has at once (if one may put it so) been fiercely for having children and fiercely for not having children. It has kept them side by side like two strong colors, red and white, like the red and white upon the shield of St. George. (Image Books Edition: 1959, p. 97)
Suddenly that night things I knew just clicked in place as we discussed Chesterton’s appreciation of St. Thomas Aquinas as “St. Thomas of the Creator”. This demonstrates the benefits of thinking about great things with wonderful people—including Chesterton.