Pastor Timothy Keller paraphrases an analogy originally used by C.S. Lewis (in his book Mere Christianity) to demonstrate the nature of sin in our hearts.

Now if you want to know if there are rats in your basement, you don't walk to your basement door, clear your throat, and say, "I think I'll go down and see if there are rats in my basement," then jiggle the knob, open the door and in a very leisurely way turn on the light, clear your throat, and walk down the steps loudly and slowly. When you get to the bottom you look around and say, "Well, what do you know: I have no rats in my basement."

If you want to know if you have rats in your basement, you sneak up to the door, silently open the door, flick on the switch, jump to the bottom of the steps, and look around and they'll all be scurrying away. And then you'll know if you have rats.

Based on this analogy, Lewis wrote:

The excuse [for most of my sinful moments] that immediately springs to my mind is that the provocation was so sudden and unexpected; I was caught off my guard [like a rat who didn't get enough warning] … Now that may be an extenuating circumstance as regards those particular acts: they would obviously be worse if they had been deliberate and premeditated. On the other hand, surely what a man does when he is taken off his guard is the best evidence for what sort of a man he is? Surely what pops out before the man has time to put on a disguise is the truth?