 It is also possible—according the the standard theory—to use “only if” to identify a necessary condition: we can say that Jonah was swallowed by a whale only if he was swallowed by a mammal, for if a creature is not a mammal, it is not a whale. Equivalent to (i) above, on this account, is the sentence “I opened the door only if I used the key”—a perfectly natural way of indicating that use of the key was necessary for opening the door.