In his heart (en th kardiai autou). Not just the centre of the blood circulation though it means that. Not just the emotional part of man's nature, but here the inner man including the intellect, the affections, the will. This word is exceedingly common in the New Testament and repays careful study always. It is from a root that means to quiver or palpitate. Jesus locates adultery in the eye and heart before the outward act. Wunsche (Beitrage) quotes two pertinent rabbinical sayings as translated by Bruce: "The eye and the heart are the two brokers of sin." "Passions lodge only in him who sees." Hence the peril of lewd pictures and plays to the pure.