Certain positive polarity items are acceptable within negated antecedents of counterfactuals, as pointed out by Baker (1970) and Horn (1970). These include temporal adverbials such as 'already' and 'still' and degree modifiers such as 'pretty':

 (1) If she were not already/still here, we would have postponed the meeting.
 (2) If she weren't pretty smart, she wouldn't have passed the exam.

Baker (1970) attributed this type of polarity reversal to the presupposition of the falsity of the antecedent associated with counterfactuals and regarded it as evidence that polarity items can be licensed derivatively by presupposition as well as entailment. However, such an approach fails to account for the asymmetry between positive and negative polarity items in this respect: (3) is unacceptable even in contexts entailing that he is not yet here; similarly (4) is unacceptable even in contexts entailing that he did not write terribly well.

 (3) *If he were here yet/anymore, we would have postponed the meeting.
 (4) *If he had written terribly well, he would have passed the exam.

In this talk I show that both the polarity reversal in (1) and (2) and the asymmetry between (1)-(2) and (3)-(4) can be explained without any special mechanisms over and above those required to account for regular licensing. They are a consequence of the lexical meaning of the polarity items involved - including their assertive and presuppositional content as well as the alternatives they are associated with - and of the semantics of counterfactual conditionals. The account I propose relies crucially on the analysis of polarity items by Krifka (1990, 1991, 1995), according to which polarity items are associated with alternatives inducing an ordering of (contextually restricted) semantic strength. Their acceptability then depends on a general condition requiring that assertions based on polarity items be informationally stronger than those based on the alternatives.

Polarity reversal in counterfactuals thus provides support for a theory of polarity licensing which is symmetric in its treatment of positive and negative polarity items and which locates the context sensitivity of licensing in the need to satisfy the general condition for greater informativity.