Obligation and Aspect

摘要

Fred must open the door' concerns Fred's obligations. This obligative meaning is turned off by adding aspect: Fred must have opened/be opening/have been opening the door' are one and all epistemic. Why? In a nutshell: obligative 'must' operates on procedural contents of imperative sentences, epistemic must' on propositional contents of declarative sentences; and adding aspect converts procedural into propositional content.