The title of this post is a remark I heard Nuel Belnap make in the question period after a talk on free will (quoting from memory).
Here, then, is a valid argument for a kind of Principle of Alternate Possibilities:
- It is not possible to rationally deliberate when one knows one that fewer than two options are possible. (Premise)
- One deliberates knowledgeably if and only if one knows all the deliberatively relevant facts. (Premise)
- It is deliberatively relevant which options are possible. (Premise)
- Therefore, if one rationally and knowledgeably deliberates, then at least two options are possible. (By (1)-(3))
(1) and (2) seem quite secure. But the opponent of Principles of Alternate Possibility may dispute (3), even though it seems very plausible to me.
In any case, (3) is clearly true in some cases. If I'm deliberating between three rescue operations, which can save, respectively, one family member, two strangers, or three family members, learning whether the third option is actually possible would, surely, affect rational deliberation (if it is possible, then it is the best choice; if it is not possible, then we have a hard choice between the first and second options). So there are at least some cases of deliberation where knowledge of what options are possible is deliberatively relevant. This isn't enough to yield (4), but it is enough to yield a weaker claim such as that rational and knowledgeable deliberation in certain kinds of real-world cases requires more than one option to be possible. If one adds the assumption that in these cases rational and knowledgeable deliberation does in fact occur, one concludes that in these cases more than one option is possible. Moreover, "possibility" here must be more than just metaphysical possibility—it must be some kind of causal possibility. (Learning that one of the rescue operations is logically impossible should affect deliberation; but learning that one of the rescue operations is causally impossible is just as relevant.) And hence, most likely, we do get something that is relevant to disputes with determinists.