This is probably Chesterton’s best insight into pretty much all modern philosophy. It explains why it is (rightly) unknowable to the common man.
Since the modern world began in the sixteenth century, nobody’s system of philosophy has really corresponded to everybody’s sense of reality: to what, if left to themselves, common men would call common sense. Each started with a paradox: a peculiar point of view demanding the sacrifice of what they would call a sane point of view. That is the one thing common to Hobbes and Hegel, to Kant and Bergson. to Berkeley and William James. A man had to believe something that no normal man would believe, if it were suddenly propounded to his simplicity; as that law is above right, or right is outside reason, or things are only as we think them, or everything is relative to a reality that is not there. The modern philosopher claims, like a sort of confidence man, that if once we will grant him this, the rest will be easy; he will straighten out the world, if once he is allowed to give this one twist to the mind.
G.K. Chesterton, St. Thomas Aquinas, p.134
Later on, he also hit’s the nail on the head with how philosophical language can collapse under the weight of it’s own doubt. Lewis addresses this head-on in The Abolition of Man. We see this kind of language meltdown most explicitly in Derrida (who came after both of these fellows).
Most modern philosophies are not philosophy but philosophic doubt; that is, doubt about whether there can be any philosophy. If we accept Saint Thomas’s fundamental act or argument in the acceptance of reality, the further deductions from it will be equally real; they will be things and not words. Unlike Kant and most of the Hegelians, he has a faith that is not merely a doubt about doubt. It is not merely what is commonly called a faith about faith; it is a faith about fact. From this point he can go forward, and deduce and develop and decide, like a man planning a city and sitting in a judgment-seat. But never since that time has any thinking man of that eminence thought that there is any real evidence for anything, not even the evidence of his senses, that was strong enough to bear the weight of a definite deduction.
-p.171
For Chesterton, even Augustine was a bit too pie-in-the-sky. He liked the very earthy Aquinas the best.
“There is an Is.” That is as much monkish credulity as Saint Thomas asks of us at the start. Very few unbelievers start by asking us to believe so little.
-p.153