Must a bulldozer be an egg? 
I recently read a forthcoming paper by Carl Brusse about conceptual change and the planet category. [1] He is "broadly in agreement with the approaches to scientific kinds argued for by Magnus", and I am broadly in agreement with him. I just want to comment on a point where he directly engages my account.

When the IAU adopted a definition of 'planet' in 2006, there were two main contenders. One was to distinguish planets as objects with enough mass that they collapse into a spheroid. The other was to distinguish planets as objects with enough mass to gravitationally dominate their orbit. Brusse calls these the egg and bulldozer conceptions, respectively. The IAU definition includes both conditions, which Brusse argues makes it "a messy hybrid".

I argue that the mass required to be an egg-planet is strictly less than the mass required to be bulldozer-planet. So the IAU definition can be rewritten in this equivalent but neater form: "A planet is an object which is not itself a star but which is massive enough to dominate its orbit around a star."

Brusse replies that "it is simply false to say that the two criteria can't come apart." He notes that it is conceptually possible for something to meet one criterion but not the other, and he offers some handwaving speculations as to how it might be nomologically possible. For example, "perhaps exotic systems with amorphous carbon/diamond bodies rigidly resisting hydrostatic collapse and tightly orbiting compact neutron stars might manage to invert the usual order of criterion satisfaction: bulldozers yes, eggs no." So, he concludes, the IAU's definition is a messy hybrid after all.

When I presented what was then work in progress on the planet category, Christy Mag Uidhir posed a similar worry during the Q&A. He asked, What about Cybertron? The fictional home of the Transformers and Decepticons is a planet, but has hard edges jutting out all over the place. It's a planet, but not an egg-planet.

My answer to Christy was that there is no reason to expect natural kind categories for the actual world to survive in exotic counterfactuals. So it matters whether the possibility of a bulldozer which isn't an egg is a genuine, nomological possibility as opposed to a mere conceptual possibility. I don't know whether Brusse's exotic systems are really physically possible or not.

Brusse concludes that "it is an empirical question whether there are any such miniature dynamical planets [bulldozers but not eggs] out there to be discovered (they would likely be beyond the reach of any foreseeable detection techniques)." That's right, but I think astrophysics gives us reasons to think that they are not very common if they do exist. If I'm right about that, then the bulldozers-not-eggs are at most edge cases. A category can be a natural kind even though it is unable to neatly handle some exotic individuals in the actual world.

For the great run of star systems, formed in the usual way, the mass required for the bulldozer criterion will be greater than the mass required for the egg criterion. If that's right, then I stand by my neatening up of the IAU definition. It's syntactic complexity is a result of the politics of its adoption, rather than any underlying incoherence.

If it turns out that Brusse's exotic systems are realized all over the place, then I'm wrong about the natural kind. That just illustrates that the natural kinds in the world are contingent. If our science gets the world wrong, then it probably also gets the kinds wrong.

Brusse adds, "on the empirical question of uptake, it is not clear that scientific language is actually converging toward the new definition, even in peer reviewed publications." I found this interesting, but it's compatible with my account. What I argued was that the IAU definition picks out one natural kind in the neighborhood of the term "planet", but I admitted that there was some arbitrariness in regimenting "planet" to mean that kind rather than one of the others in the neighborhood. The point matters more to Brusse, who is interested in the "conceptual lineage" of terminology rather than in natural kinds. As I say, I'm interested in categories rather than in words.


[1] Planets, pluralism,and conceptual lineage
in Studies in History and Philosophy of Modern Physics
DOI 10.1016/j.shpsb.2015.11.002i

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