The Incarnation is mentioned only a handful of times within the work, but it is of fundamental importance to Chesterton. The Incarnation, in Christian doctrine, is when one of the divine Trinity takes on a true, human body in the person of Jesus of Nazareth. The doctrine of the Incarnation asserts that Jesus Christ was both true God and true man, two natures united in a single person. In the Incarnation, God breaks into human history.

According to classical philosophy, and the Catholic tradition that incorporated that philosophy into its own teachings, every human being who has lived or ever will live has a definite nature. This nature is immutable and does not and cannot change by force or by choice; it is shared by every race, at every point in history, and although customs and civilizations may change, what actually makes a human being a human being does not. This means that we can trace what is common to all people throughout history.

Philosophy literally means “love of wisdom” in Greek. It is the desire and search for wisdom and for what is true and essential. Philosophy is an act performed by the human intellect. Since human nature is shared by all people, regardless of their beliefs, philosophy is common to all humans throughout history.