S. defines semantics as the study of propositions and their relations to possible worlds, whereas he characterizes pragmatics as the study of linguistic acts and the contexts in which they are performed. He assigns two tasks to pragmatics: (i) to define interesting types of speech acts and speech products; (ii) to characterize speech contexts that co-determine what propositions can be expressed by a given sentence. According to S., 'presuppositions' are the most important aspects of a context. S. claims that 'reference' can only be described pragmatically.