The concupiscence of the eyes touches the soul at a higher level than that of the flesh, and is consequently even more subtle and dangerous. Everyone can distinguish sins of the flesh, and most people endeavor to keep themselves from any serious entanglement with them, but it is quite possible to become considerably involved in the concupiscence of the eyes without being in the least aware of the fact.

The Elements of the Spiritual Life: A Study in Ascetical Theology (1960), p. 104