I found the following in "The Wisdom of Robert Collier", I really like it.
The devil once held a sale and offered all the tools of his trade to anyone who would pay their price. They were spread out on the table, each one labeled: hatred, and malice, and envy, and despair, and sickness - all the weapons that everyone knows so well.
But off on one side, apart from the rest, lay a harmless looking, wedge-shaped instrument marked "Discouragement." It was old and worn looking, but it was priced far above all the rest. When asked the reason why, the devil replied:
"Because I can use this one so much more easily than the others. No one knows that it belongs to me, so with it I can open doors that are tight bolted against the others. Once I get inside I can use any tool that suits me best."