Of course I have primitive mental processes, including animal instincts, biases, and inclinations to think fallaciously. The "reptilian" part of the human brain is relatively primitive, as in it's much older than the cerebral cortex.

When we think critically rather than intuitively, though, we exercise our personhood and our higher cognitive and social functions (logic, philosophy, science, etc). When we defer to emotions, instincts, and social power dynamics, we're thinking like primates, not like people. We betray our higher calling.

Indeed, Paul's talking about the inner conflict which stems from the fact that the human brain evolved in stages. The human brain isn't entirely unified. The cerebral cortex has some veto power and the ability to analyze alternatives, but the gut exercises great control over behaviour, which is why it takes much discipline to resist our appetite for sugar, for example. Augustine made much of this inner conflict too.

Paul makes a hash of it, of course, by moralizing the inner struggle and by presupposing that God and the devil are in control of different sides of the self. No, the whole thing is evolutionary and amoral. We add the moral values in our judgment of what kind of people we want to be.