That declaration also in the Gospel, If I had not come and spoken unto them, they had not had sin; but now they have no excuse for their sin, renders it manifest and patent to all who have a rational knowledge of how long a time man is without sin, and from what period he is liable to it, how, by participating in the word or reason, men are said to have sinned, viz., from the time they are made capable of understanding and knowledge, when the reason implanted within has suggested to them the difference between good and evil; and after they have already begun to know what evil is, they are made liable to sin, if they commit it. (Book I, Chapter 3, Part 6)

Innate to reason is the ability to distinguish between good and evil.

To distinguish between these is also to be able to choose between them.

When we can discern good from evil and fail to choose the good, we have rejected our own fundamental nature and distanced ourself from God.

The image is the Tree of Knowledge