At the heart of semantics in the twentieth century is Frege's distinction between sense and force. This is the idea that the content of a self-standing utterance of a sentence S can be divided into two components. One part, the sense, is the proposition that S's linguistic meaning and context associate with it as its semantic interpretation. The second component is S's illocutionary force. Illocutionary forces correspond to the three basic kinds of sentential speech acts: assertions, orders, and questions. Forces are then kinds of acts in which propositions are deployed with certain purposes.
There are at least five reasons for positing Frege's distinction, which we can discern in Searle's Speech Acts (1969) and other writings:
R1: It seems we ought to analyze assertion in terms of belief: assertions are acts in which we utter sentences aiming to manifest our commitment to belief states. Belief states are truth-apt. Therefore, the primary truth-bearers are prior to assertion. Furthermore, beliefs are propositional attitudes, states that comprise an attitude component – characteristic of belief – and a content. It seems reasonable then to equate the primary bearers of truth with the contents of belief states, and to claim that these contents are propositions. Thus the content of an assertion involves two components: a propositional content – the object of the belief that the assertion is a manifestation of – and a force – the act type which is that of committing oneself to a certain belief.