9 The mischief is much more trifling, and the danger less, when the limbs are wounded with a sword. The cure is easy where the wound is manifest; and when the medicament is applied, the sore that is seen is quickly brought to health. The wounds of jealousy are hidden and secret; nor do they admit the remedy of a healing cure, since they have shut themselves in blind suffering within the lurking-places of the conscience. Whoever you are that are envious and malignant, observe how crafty, mischievous, and hateful you are to those whom you hate. Yet you are the enemy of no one's well-being more than your own. Whoever he is whom you persecute with jealousy, can evade and escape you. You cannot escape yourself. Wherever you may be, your adversary is with you; your enemy is always in your own breast; your mischief is shut up within; you are tied and bound with the links of chains from which you cannot extricate yourself; you are captive under the tyranny of jealousy; nor will any consolations help you. It is a persistent evil to persecute a man who belongs to the grace of God. It is a calamity without remedy to hate the happy.
Source: The Treatises of Cyprian (New Advent)