No barbarian people wages so relentless a war as do the wicked thoughts that lodge within the soul and the disordered passions…This is easily understood, since the first wave of enemies attacks us from without, and the second wave makes war against us from within. Without fail, one can observe that internal evils are more disastrous and pernicious than external ones…Nothing is more deadly to bodily health and strength than the infirmities that develop within it–cities suffer less from foreign wars than from internal dissent. Likewise, the soul has less to fear from the snares laid for it in the world than from the illnesses whose seed the soul itself has sown.
~St John Chrysostom
Prayerful Life 