If one converts surface filters into context free rules, one has to introduce new features. These features are strictly nonlexical, and their distribution is predictable from the distribution of the lexical features. Now, given a (feature based) context free grammar, we ask whether one can identify the nonlexical features. This is not possible; however, the notion of an inessential feature offers an approximation. To arrive at a descriptive theory of language one needs to eliminate all the inessential features. One can measure the complexity of a language by the complexity of the formulae needed to define the distribution of inessential features.