According to St. Augustine, the inner man is differentiated from the outer man by his ability to reason. The ability to reason makes a human person different than animals. This abstract knowledge from the fourth century presented by St. Augustine is now backed up by science.
According to the neuroscientist, Paul MacLean in his book The Triune Brain in Evolution, the human brain has a reptilian brain, a limbic system, and a neocortex. The neocortex specifically relates to human beings and their ability to think abstractly. The neocortex can be associated with St. Augustine’s inner man. “As we ascend, then, by certain steps of thought within, along the succession of the parts of the mind, there where something first meets us which is not common to ourselves with the beasts reason begins, so that here the inner man can now be recognized” (de Trinitate XII, viii).
The inner man, or the neocortex part of the human brain, has the ability to make wise decisions. For example, the ability to forgo instant pleasure and live a self-disciplined life is a higher function of reasoning that animals to do not possess. This ability to restrain oneself is associated with St. Augustine’s masculine part of the human person, which is made in the image of God. This image of God can become tarnished as the human person acts with lower reasoning skills and less wisdom. The reptilian brain is associated with aggression and dominance as well as impulsiveness. I believe that the vice of pride is rooted in the reptilian brain. I believe this is true because the reptilian brain is associated with survival at all costs. Aggression and dominance are based in fear. Pride is simply a psychological blanket to cover fears and insecurities.
St. Augustine states that when the human mind does not act with wisdom, the human person reflects the image of beasts instead of the image of God. This likeness to beasts involves the reptilian brain feeding one’s attraction to corporeal things. The mindless pursuit of corporeal things is associated with the bad fruits of lust, power, and addictions of all kinds. Functioning at the lower levels of reason tend to gradually transform the human person into more of an animal than a reflection of divinity. Thus, mortal sin is the loss of connection with God and the morass of a beastly life.
St. Augustine writes, “For as a snake does not creep on with open steps, but advances by the very minutest efforts of its several scales; so the slippery motion of falling away [from what is good] takes possession of the negligent only gradually, and beginning from a perverse desire for the likeness of God, arrives in the end at the likeness of beasts” (de Trinitate XII, xi). Thanks to 20th century science, now we know the true meaning behind the snake that fooled Adam and Eve.