to build a model for Z and given X the state of the world which is exactly the kind of sensory model that we like to build so we get a simplification happening over here we had ZN conditioned on everything that's the standard Bayes rule with conditioning but then we made an assumption there's not generality but we made an assumption that Z and is independent of the other Z's if we already know the state X and then this here has simplified then again the bottom we don't worry about there's no X in it we can compute the top part