Thomas Aquinas, Compendium of Theology

The harmonious integrity of the original state depended entirely on the submission of man's will to God. Consequently, as soon as the human will threw off the yoke of subjection to God, the perfect subjection of the lower powers to reason and of the body to the soul likewise disintegrated. As a result, man experienced in his lower, sensitive appetite the inordinate stirrings of concupiscence, anger, and all the other passions. These movements no longer followed the order set by reason but rather resisted reason, frequently darkening the mind and, so to speak, throwing it into confusion. This is that rebellion of the flesh against the spirit which Scripture mentions. For, since the sensitive appetite, like all the other sense powers, operates through a bodily instrument, whereas reason functions without any bodily organ, what pertains to the sensitive appetite is rightly ascribed to the flesh, and what pertains to reason is attributed to the spirit. This is why substances that are without bodies are commonly called spiritual substances.