Actuality Entailments and the Argument from Collapsed Duality

Modal expressions—expressions like ‘must’ and ‘can’—figure importantly in our ability to think and talk about things that are non-actual: people who are not actual (e.g., ‘There could have been a Tom Collins’1), events that did not happen (e.g., ‘He could have been spreading rumors’), ways in which events that happen do not happen (e.g., ‘Rome could have been built in a day’), etc. These expressions are not, in general, required to represent what the actual world is like. Recently, however, certain environments have come to light in which these expressions (among other expressions that appear to be modals) are required to represent what the actual world is like. Consider, for example, that the following two sentences bear interpretations on which they differ with respect to whether it is entailed that we took the train: