Day 324: The Ninth Commandment

The "pure in heart" are promised that they will see God face to face and be like him. Purity of heart is the precondition of the vision of God. Even now, it enables us to see according to God, to accept others as "neighbors"; it lets us perceive the human body - ours and our neighbor's - as a temple of the Holy Spirit, a manifestation of divine beauty. CCC 2519

In today's reading, the Catechism introduces the Ninth Commandment. Lust is at the heart of this: lust of the flesh, lust of the eyes, and pride of life (CCC 2514). The eyes are said to be windows to the soul, but they can also lead us to the lust of the flesh (Mt 5:28). This is extended further to the heart, which is "the seat of moral personality" (CCC 2517), wherein many of our struggles with sin lie (Mt 15:19). It is also can be said that there is where our battle is waged to be "pure in heart" so that we can "see God" (Mt 5:8).

The Catechism Compendium summarizes what is required and also what is forbidden by the Ninth Commandment:

The ninth commandment requires that one overcome carnal concupiscence in thought and in desire. The struggle against such concupiscence entails purifying the heart and practicing the virtue of temperance. The ninth commandment forbids cultivating thoughts and desires connected to actions forbidden by the sixth commandment. (#527-28)

The Catechism Companion Vol III has some good commentary on this:

Commandments nine and ten are recapitulations of the Sixth and Seventh Commandments. We have an attraction to what is not good for us. Lust of the eyes is a type of greed seeking to possess another thing, to be able to use it. Lust of the flesh is wanting to possess another person, wanting to use another person. They come from the same place in the heart, the impulse that says, "I'm willing to take what is not mine." ... Temperance can help us develop self-control, to say no to our impulses. (p. 164)