On the basis of evidence from phenomena involving tense and aspect in English as well as certain syntactic distinctions arising in complement constructions, C. argues that it is justified to treat presupposition in a grammar. A number of hypotheses about the form of the representation of presuppositions, are formulated. Complex sentences with factive verbs in their matrix clause are assigned an underlying representation with the presupposed complement sentence to the left of the matrix and dominated by the abstract performative sentence 'We (inclusive) 'assume' that S'.