Abstract

I contend that the Conditional Excluded Middle -- > ) v > )--is a valid axiom which should be accepted as part of an optimal counterfactual theory. With CEM, our formal theory can do maximal justice to intuitions from ordinary language, such as supporting the intuitively-correct equivalence between > ) and > ), while carrying over an important axiom from ordinary logic. ;My defense of CEM is conducted within a semantics of possible worlds, developed primarily by Stalnaker and Lewis. Essentially, I adopt the position that a counterfactual > ) is true just in case is true in the unique possible world which is the most similar to the actual world except that is true there. This truth condition will insure CEM, but it depends on an important assumption that such a unique possible world can always be realized. Two main complaints against Stalnaker's Assumption are that sometimes there seems to be more than one equally close closest world, while at other times there seem to be ever closer worlds without limit. I address the first complaint mainly by pressing an intuition that counterfactual antecedents purport to refer to a unique world, and then by defending the position that such an intuition can be shown to be reasonable. As to the second complaint, I argue that deep analysis shows that the "no-limit" cases never occur among clear, consistent counterfactual antecedents. ;As a major part of my thesis, I analyze the English modals "would" and "might" which occur in standard counterfactual consequents. I claim that "would" signals what is the case in a unique counterfactual situation, and that "might" signals what is possibly the case in a similar situation. This latter definition, the one for "might" in counterfactuals, has been developed to show that Lewis is incorrect in his claim that counterfactuals featuring "might" cannot be adequately defined in a system with CEM. And, I demonstrate that when "would" and "might" are interpreted in the way I suggest, the position of CEM is strengthened and shown to be intuitively soothing