The purpose of this study is to analyze Root Modality must in terms of possible worlds semantics and speech acts theory. The possible worlds semantics is proposed by Kratzer (1981, 1991, 2012). Speech acts theory is developed by Searle (1969, 1979). “Indirect speech acts” is one of most well-known theories in the field of linguistics. Modality is ambiguous. For example, the English auxiliary Root must can have two basic meanings, one is “obligation” and the other is“ invitation”. Basically, modality refers to the mental attitude of a speaker and the condition of facts or matters. This study attempts to determine what utterance situation for invitation must is.
The first part of this essay offers the renewal modal logic definition of must, revised from Goda (2015). From the viewpoint of possible worlds semantics, this study suggests that must has one meaning, “obligation”. In the second part, we argue that background situations are deeply connected to our conversations by using pragmatics in our study. We can utter a must-sentence for two separate functions, the one is “deontic usage” and the other is“ invitation usage”. We find there are some differences between deontic must and invitation must by analyzing speech acts. When the invitation must works as a function of invitation, must requires some conditions and steps which change it from deontic must in indirect speech acts theory. This is called hearer’s implicature.
This study concludes the following. Actually, must-sentence are regarded as “assertives”. When the source of obligation is relevant to speaker’s social power, must tents to be deontic. The utterance situation of invitation must is that speaker believes the acts should be accomplished for listener, and the speaker expects the listener’s agreement. On the other hand, in deontic must situation, in fact the speaker just knows the necessity of accomplishing it.