 A significant complication is that in ordinary conversation, the context matters with regard to which proposition is expressed with a given sentence. For example, ‘Pat is asleep’ can be used at one time to express a true premise, and at another time to express a false premise. A certain speaker might use ‘I am tired’ to express a false proposition, while another speaker uses the same sentence at the same time to express a true proposition. What counts as being tired can also vary across conversations. Context sensitivity, of various kinds, is ubiquitous in typical discourse. Moreover, even given a context, a sentence like ‘He is bald’ may not express a unique proposition. (There may be no referent for the pronoun; and even if there is, the  vagueness  of ‘bald’ may yield a range of candidate propositions, with no fact of the matter as to which one is the proposition expressed.) Still, we can and often do use sentences like ‘Every circle is an ellipse’ and ‘Thirteen is a prime number’ to express premises of valid arguments. To be sure, mathematical examples are special cases. But the distinction between impeccable and risky inferences is not limited to atypical contexts in which we try to think especially clearly about especially abstract matters. So when focusing on the phenomenon of valid inference, we can try to simplify the initial discussion by abstracting away from the context sensitivity of language use.