Logical necessity: must, be bound to (BrE), have to (AmE)
The second type of epistemic modal certainty is that of logical necessity, meaning ‘it is necessarily the case that the assertion is true’. Must is the modal most used in BrE to express strong conviction based on deduction or inference from evidence, which may or may not be stated. The concert must be over might be said, for instance if the speaker sees that the lights are off or the concert hall is closed. When B contradicts A saying Oh no. He must be at least fifty, must indicates that B does not actually know Bill’s age but is convinced that Bill is fifty at least. With will and must, the speaker does not admit any possibility of the assertion not being true. For this reason, adding ‘but it/he may not be’ would result in a contradiction: ‘He must be at least fifty,*but he may not be’ is contradictory.
Must in general is far more frequent in BrE than in AmE, but the logical-predictive meaning of must in AmE is more common than its obligation meaning as in I/you must leave now. For both meanings AmE prefers the lexical verb have to. This alternative is also making headway in spoken BrE, since it is more objective than subjective must with the meaning of logical necessity, and also with that of obligation:
The key must be in your pocket. (BrE and AmE)
The key has to be in your pocket. (AmE and with younger speakers of BrE)
The key is bound to be /is sure to be in your pocket. (BrE)
If Jane is Pat’s sister and Jill is Jane’s daughter, Pat must be Jill’s aunt. (This example illustrates a strict interpretation of logical necessity (‘this is the only possibility there is’.)
