It is proposed that “must” and “have to” express aggregative modalities, i.e., they represent the combined effects of various types of inducements, including legal and moral obligations, the individual’s own commitments and decisions, practical necessity, etc. The logic of “must” and “have to” is shown to differ from that of pure deontic modalities, such as “have an obligation to”. Aggregative predicates can be used to clarify the processes in which we combine different types of requirements in order to determine what to do. This analysis also makes it possible to avoid the common deontic paradoxes, such as Ross’s paradox, for some of the deontic predicates.