 The Self-Defeat Question: Suppose that some thesis according to which a certain sort of conceivability entails a certain sort of possibility—call it (CP)—is true. Is it then conceivable that (CP) is false? If that is conceivable, or so Howell (2008) and Mizrahi and Morrow (2015) have argued, then, by (CP), it is possible that (CP) is false. However, if (CP) is possibly false, (CP) is actually false. For a counterexample to the entailment relation falsifies the entailment relation. Thus, if we can conceive of (CP) failing, it follows that (CP) is self-defeating.