  Gödel invented (in 1938) the method of inner models by defining the minimal inner model L.  This model is defined just as V is defined except that at successor stages instead of taking the full powerset of the previous stage one takes the definable powerset of the previous stage, where for a given set X the definable powerset Def(X) of X is the set of all subsets of X that are definable over X with parameters from X: