This is a pre-print version of J. Cantwell (2008) “Changing the modal context”, Theoria, Volume 74, Issue 4, Pages: 331351. Changing the Modal Context John Cantwell Abstract Conditionals that contain a modality in the consequent give rise to a par- ticular semantic phenomenon whereby the antecedent of the conditional blocks possibilities when interpreting the modality in the consequent. This explains the puzzling logical behaviour of constructions like “If you dont buy a lottery ticket you cant win”, “If you eat that poison, it is unlikely that you will survive the day and If you kill Harry you ought to kill him gently”. In the paper it is argued that a semantic version of the Ramsey Test provides a key in the analysis of such constructions. The logic for this semantics is axiomatized and some examples are studied, among them a well-known puzzle for contrary-to-duty obligations. 1 Introduction 1.1 The problem Consider: (1) If you don’t buy a lottery ticket you can’t win. (2) If you eat that poison, it is unlikely that you will survive the day. (3) If you kill Harry, you ought to kill him gently. (4) If Jane falls off the cliff, then if she is still alive at the end of the day, she can fly. These four sentences illustrate a general and rather puzzling phenomenon. There is a way of interpreting these sentences—I think it is the dominant interpretation— in which modus tollens fails: it doesn’t follow from the truth of the conditional and the falsity of its consequent that the antecedent is false. It is true that you can’t win if you don’t buy a lottery ticket, but it is also true that you can win on the lottery (for if you buy a ticket you might win) yet it doesn’t follow that you will buy a lottery ticket. It is true that it is likely that you will die before the morning if you eat that poison, but it isn’t likely that you will die before morn- ing (for most likely you will not eat the poison and you are otherwise healthy), yet it doesn’t follow that you will not eat the poison. It is better to kill Harry 1