Doubt is cast on the belief that non-restrictive clauses arise from independent sentences and, hence, have their own performative verb (as suggested in J.P.Thorne (1972: 'On non-restrictive relative clauses')), because on the basis of that hypothesis one cannot explain why one would utter a non-restrictive relative clause instead of an independent sentence. Therefore L. tries to characterize the function of non-restrictive relative clauses in discourse.