A central question for any theory of the interpretation of conditionals is what can be held constant and what must bce given up on the face of a counterfactual supposition. This chapter brings grammatical evidence to bear on the question from polarity reversal, the phenomenon where positive polarity items can exceptionally, albeit systematically, appear in the scope of negation in the antecedent of counterfactual conditionals. Taking polarity sensitive expressions to be associated with alternatives and to give rise to scalar assertions, it shows that polarity reversal can result in scalar assertions because in making a counterfactual assumption any contextual entailments are given up once the information that gives rise to them is revised. The analysis reveals the role that contextual information tied to presuppositions plays in determining a particular type of dependency between facts.
