Sunday, August 27, 2006


Sorry Pluto… And Some Thoughts About Categorization

Because semantics and categorization are key themes in document engineering and in the courses I teach, I've been flabbergasted by much of the recent reaction to the "demotion" of Pluto from planethood to the inferior status of "dwarf planet"” by the International Astronomical Union. The IAU recently passed a resolution that defined a planet within our solar system as:

A celestial body that is (a) in orbit around a star, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

Because Pluto doesn't satisfy the third requirement, it no longer is classified as a planet. This has generated a great deal of news and caused lots of people to get upset. A typical headline is "Pluto's demotion has schools spinning" -- elementary school science teachers just don't know what to teach. And the widow of the astronomer who discovered Pluto in 1930 says she is all shook up.

But "Pluto's place is safe with astrologers" and that's crucially important to me because apparently Pluto is the most important planet for Scorpios and I am a Scorpio.

These stories are pathetically amusing and that alone makes them interesting, but what really intrigues me is how they illustrate how people understand categories. "Planet" is a category with profound historical and cultural importance, and because of the IAC resolution, we get to witness a very clear and sudden shift in how that category is defined.

For millennia we earthlings have had a notion of planet as a "wandering" celestial object, but because we only knew of planets in our own solar system, we could define "planet" by enumeration. Very few categories can be understood that way, that is, by making an exhaustive list of their members. But once we acknowledge the existence of planets outside our solar system, the set of planets becomes unbounded, and the lack of a definition becomes apparent. Then we can have arguments about the definitions, and hence biases of one kind or another get built into the categorization.

The popular reaction to this new way of understanding "planet" by extensional definition rather than by enumeration suggests that many people are living with the delusion that there is an objective reality in which categories and definitions are objective and unchanging. And it is scary to read that the IAC members gave serious discussion to the likely impact their new definition would have on elementary school science. I thought progress in science, scientific revolutions, creative destruction and all that meant that we should look forward and not worry about the “installed base” of people with a sixth-grade science education.

-Bob Glushko

Oops, I was a little careless in my use of terminology and said "extensional definition" when I meant "intensional." My colleague Eric Wilde points out that I meant to say

The popular reaction to this new way of understanding "planet" by intensional definition rather than by extension suggests" ...

"Intension" and "extension" are nicely explained in Wikipedia.

-Bob Glushko
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