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.
2 comments:
I am not sure about (1). Suppose a mine has collapsed, trapping some miners. I am in charge of the rescue. The mine has two entrances. I know (somehow) that one of the entrances is blocked, but not which one. I still have to deliberate which mine entrance to head down; I would probably deliberate on the basis of which entrance, if open, would provide best access.
This is not a perfect example. But the general idea is that you may know that only one thing is possible in the metaphysical sense, but several things are possible in the epistemic sense, and have to deliberate in those circumstances. Presumably that is how the determinist sees matters.
You may be right, though this seems to be a case where you're deliberating which mine entrance to try to head down, and here both options are possible.
But instead of making this response, I should modify (1) to avoid such controversy:
(1*) If you know that there is only one possible option and you know which one it is, you cannot rationally deliberate.
The rest of the argument goes through.
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