The research on grammaticalisation has shown that morphosyntactic change is not necessarily unidirectional as originally postulated. Semantic change, however, tends to be unidirectional: the meaning undergoes subjectification, shifting from the speaker-external to the speaker-internal perspective, never the other way round. The analysis presented in this paper focuses on two modal verbs, English must and Polish musieć, and it shows that, semantically, both verbs developed in the same direction. To account for the changes, the author uses the Invited Inferencing Theory, as put forward in Traugott and Dasher (2005). The data presented clearly support the thesis that unidirectionality does hold at the semantic level.