that imagine you deliver the sensor to somebody and they lock the door and nobody has the key anymore then your sensor can't know that because you didn't know that ahead of time what you really can build is the opposite distribution that causal one given a situation what am I going to measure and so that's what's be available to us and that's what we have to use but it's not what we want the reason about the world so we'll use Bayes rule to get what we want which is the opposite thing going from the distribution that we know how to model