The contrasts Jesus makes between his teaching and the Law are not intended to contradict what the Law requires. Rather, the content following "but I say to you" draws from other parts of the Law to rightly interpret the referenced portion. For example, "Do not commit adultery" and "Do not covet/lust" are both commands in the Decalogue (Exod. 20:14, 17). as the control that indexes when the prohibition against adultery (Exod. 20:14) has been violated (Matt 5:28). - Paul T. Sloan, Jesus and the Law of Moses: The Gospels and the Restoration of Israel within First-Century Judaism