…is not man’s heart, do you not suppose, “an abyss”? For what is there more profound than that “abyss”? Men may speak, may be seen by the operations of their members, may be heard speaking in conversation: but whose thought is penetrated, whose heart seen into? What he is inwardly engaged on, what he is inwardly capable of, what he is inwardly doing or what purposing, what he is inwardly wishing to happen, or not to happen, who shall comprehend?
[Augustine, Expositions on the Psalms 42.13]
