H. and K. define presuppositions as the conditions that must be satisfied for the communicative function of an utterance to come about (excluding conditions that must hold for an utterance to come about at all, e.g. the condition that there should not be any physical impediments to communication). The communicative function of an utterance is defined as that part of the intended function of the utterance which is explicitly linguistically indicated. (This definition serves to exclude illocutionary forces from their scope of investigation). A 'condition' of the type mentioned is said to be 'satisfied' if it belongs to the background assumptions of both the speaker and the hearer. Consequently, presuppositions are mainly a matter of beliefs. This leads H. and K. to rely heavily on S.R.Schiffer's (1972: 'Meaning') notion of 'mutual knowledge' in constructing their descriptive apparatus for presupposition. That descriptive apparatus is then used to define cases of presupposition failure, i.e. speech events in which a certain condition does not belong to the background assumptions of either the speaker or the hearer, or in which the speaker or the hearer assumes that it does not belong to the other's background assumptions, etc..