You viper's brood, how can you say what is good when you are bad? For from what overflows the heart the mouth speaks.
But when the unclean spirit goes out of a man, it wanders through waterless regions looking for a place to rest, and finds none. Then it says: I will return to my house that I came from; and it comes and finds it free and swept and furnished. Then it goes and picks up seven more spirits worse than itself, and they go and settle there; and the end for that man is worse than the beginning. Thus it will be also with this evil generation.
~from The Gospel of Matthew, translated from the Greek by Richmond Lattimore
This explains, for instance, why conquering one's addictions through the exercise of will power is not enough. And why the resulting self-satisfaction is fatal.