The rule of gradualness.

Do you need to instill a person with an idea or make them agree to everything? More than simple. All it takes is some time and patience. It is necessary to gradually inculcate the same idea into the subject's head day after day, and after a while they themselves will begin to think so. An excellent and illustrative example is abuse. When one blames the other for all their troubles: "Yes, it's your fault that I'm an alcoholic." If you tell someone every day that they are a fool, then sooner or later they will start to think so, too.