Above all, we must be especially alert against the beginnings of temptation, for the enemy is more easily conquered if he is refused admittance to the mind and is met beyond the threshold when he knocks. — Someone has said very aptly: Resist the beginnings; remedies come too late, when by long delay the evil has gained strength. First, a mere thought comes to mind, then strong imagination, followed by pleasure, evil delight, and consent. Thus, because he is not resisted in the beginning, Satan gains full entry. And the longer a man delays in resisting, so much the weaker does he become each day, while the strength of the enemy grows against him.
– Thomas a Kempis –
from The Imitation of Christ