The first source of sin is error in the understanding, which is the natural guide and counselor of the will. Consequently, the chief endeavor of the devil is to darken the understanding, and thus draw the will into the same error. Thus he clothes evil with the appearance of good, and presents vice under the mask of virtue, that we may regard it as a counsel of reason rather than a temptation of the enemy. When we are tempted to pride, anger, ambition, or revenge, he strives to make us believe that our desire is just, and that not to follow it is to act against the dictates of reason. Man, therefore, must have eyes to perceive the perfidious hook which is concealed beneath the tempting bait, that he may not be misled by vain appearances.
--Louis de Granada, The Sinner's Guide
