What do you mean, "impossible?"

When presented with alternative proposals — for example, arguments about what kind of constitutional arrangement the United States ought to have — how can one go about evaluating arguments about the likelihood or practicability of the alternatives? How do you evaluate an argument that because one proposal or set of proposals is impossible, an alternative is more likely to be true or more desirable? You've probably encountered arguments that take the form:

The three alternatives are X, Y, and Z. X and Y are impossible, therefore Z is more probable (or desirable) than X or Y.

I'd try to reduce that to symbolic notation, but I'm not sure how to go about writing the appropriate HTML, and in any event once I'd gotten started we'd find that those assertions contain some ambiguities. I'd try to analyze. It would get complex. You'd get bored. (I probably would, too.) The resulting mess would illustrate a problem — there are a lot of invalid or at least unsound arguments out there about the possibility or impossibility of this, that or the other.

The point of all this is to draw your attention to Lawrence Solum's working paper, Constitutional Possibilities. In that paper, Solum describes a "conceptual toolkit" one might use to evaluate arguments that either implicitly or explicitly depend on claims about what is or is not possible. Solum argues that certain conceptual tools that have been useful in economic theory (such as the notion of second best), philosophy (particularly possible worlds and related tools of modal logic, where "modal" means "about possibility"), and other theoretical fields can improve the articulation and evaluation of arguments among constitutional theorists.

Solum also recommends three "Standards for Modal Constitutional Arguments": First, when one makes claims about possibility or impossibility, one should articulate the kind of possibility one is referring to, because there is more than one. Second, when one rejects a proposition based on its impossibility, one should explain why. Finally, when one argues against an alternative on the ground that it is not feasible, one ought to show that one's preferred option is feasible.

I'm not a constitutional theorist, but it seems to me that the tools described in Solum's 26-page draft could increase the clarity and rigor of any argument that hinges on assertions or assumptions about possibility, not just those in constitutional theory. If you're planning to make arguments about what might be or what can't be, don't let dispassion for constitutional theory deter you from reading Solum's paper.