I had the good fortune to have a long talk with Scott Rosenberg yesterday. Putting together two pieces of the book, he asked me how Eleanor Rosch’s prototype theory applies to the planets, since I’ve been pointing to the row over Pluto as an illustration of our ancient desire to think there is a “real” order of the universe.

I stumbled around for an answer. I think now maybe the right answer is that we take one of the “normal” planets as our prototype — Jupiter is too big, Venus is too hot, Saturn has fancy-dancy rings — and use that as our idea of what a “real” planet should be like. We therefore want to call similar objects circling the Sun planets, in which case we’d end up with about 900 of them (according to one scientist I talked with). But, we also don’t want to have that many because we’ve been taught that planets aren’t a type so much as a particular set. Sort of like the Kennedy clan: No matter how much you make your hair look like JFK’s, even if your initials are JFK – are you listening, John F. Kerry? – you are not a member of  the Kennedy clan.

Which is to say that prototype theory isn’t the only theory we need. We sometimes need precise, Aristotelian definitions, as when we have to decide whether a boat trailer needs a vehicle license plate or not. And sometimes we have groups the membership of which is not governed by prototypicality.