Book 1 - Knowledge of God the Creator
Chapter 15 - Of Human Nature, the Soul, Image of God, Free Will
Sections 6-8 - Opinions of the philosophers on the soul criticized in view of the fall of Adam
Section 6 - The soul and its faculties
Only Plato saw the soul's immortality and spirituality rightly.
(It doesn't just animate the body.)
Our soul gives a sense of virtue, an aspiration to happiness in God.
Section 7 - Understanding and will as truly fundamental
These two make up the soul.
The understanding governs; the will executes.
Section 8 - Free choice and Adam's responsibility
Adam fell "solely by his own will."
His choice was free to obey or disobey, though he was designed to obey.
He had to pervert himself to disobey.
He was free to choose the good, but had not the will to do it.