The Epistle of the Apostle Paul to the Church of Rome is a letter fundamentally centered on the doctrine of Justification and with it, the faith and the commandment of love. The doctrine of Justification is one of the foundations of the catechesis inside the Catholic faith and the origin of the schism between Protestants and Catholics.
Inside this context Paul affirms: For those who live according to the flesh set their minds on the things of the flesh; but those who live according to the spirit, set their minds on the things of the spirit; Romans 8:5.
Paul marks the difference between the natural man and the spiritual man.
The spirit, or to say the divine substance, is the difference between the natural man and the spiritual man, we can say that there are orientations of the mind, choose modes: we sin or we live righteously, we are destructive or edifying, we love well or we love selfishly.
When we are selfish, we are only interested in our own good, in to have more possesions and power. When we are not self interested we are not only interested in our well but also in those around us.
We choose between God and money. We serve our own greed or we work to improve the world.
For the natural man, the spiritual life has no meaning for this type of man the divine does not exist. For the natural man holiness is crazy. Natural man always asks: What you gain? What is it for? For this type of man everything has a price, a gain, an utility. And holiness can not be measured in these terms, ultimately becomes crazy.
Holiness or spiritual life can only be understood in these terms: improve oneself, progress, minimize suffering and error, find the perfect, improve the world, help our neighbor.
The spirit and the flesh are approaches to life, different things are shared. Sin and Wisdom are attractive poles for the mind. The friendship between the natural and spiritual man can only occur in utilitarian terms, they are different languages, different worldviews. We are responsible for who we are.