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By Robert McHenry : BIO| 25 Aug 2020
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Pluto is no longer a planet. That Pluto was a planet turns out to have been a matter of great public interest, if the media coverage of its demotion is a fair indication. It was on the front page of my local newspaper yesterday (of course, so was an article about the inane television program "Survivor" -- something about racing teams, or racial teams, or whatever), and a headline in today's edition refers to "beloved Pluto." It's been discussed over and over on television news, and, of course, it's been blogged.

It's hard to divine the source of this fascination. In part (like the "Survivor" article) it simply reflects the judgment of a lot of news editors. Maybe the pause in the shooting war in Lebanon left them with time and space to kill. Maybe it's just August and the traditional journalistic silly season, brought on, as we all know, by the annual rising of Unsirius, the Dog-and-Pony Star.

But in part I suspect that the interest is genuine and stems from a common confusion of thought, the error of believing that categories are real. This was one of the great intellectual battles of medieval philosophy, that between realism and nominalism. The realists held that "universals" such as categories had an independent existence, apart from and in some sense superior to actual objects. Nominalists held that universals are only mental constructs. By and large, the nominalists won the argument but little more. Like the flatness of the Earth, realism crops up in our uncritical thinking still.

A realist of this kind is apt to assume that "planet" is a natural kind, that it's something with certain essential characteristics and that the properly equipped observer just knows one when he sees it. A tree is a tree, a frog is a frog, a planet is a planet, and a good cigar is a smoke. End of story. And so, when it is suggested for some reason that Object X, which we always knew to be a tree or a frog or a planet, isn't one, the suspicion arises that there's something, well, suspicious going on. Whaddya mean Pluto isn't a planet? Of course it is. Always was, always will be.

So what happened to Pluto? How has it changed, that its status should be questioned and people all over the world apparently be concerned? In fact, Pluto has not changed at all. It's still the undersized, slightly oddball object it always was -- undersized and oddball, that is, in comparison with some characteristics of some other objects that we accept as indisputably planets.

I learned how to think about categories from John M. Ellis: (I paraphrase) A category is not a collection of like things; it is a collection of unlike things that we choose to consider like for some particular purpose. In other words, categories are tools for thinking, made and open to remaking by humans.

Look at it this way: There is this star, and around it revolve a huge number of objects. Four of those objects are significantly larger than any others, so it seems natural to think of them as members of a category, perhaps one labeled "really big things revolving around the Sun." The question then arises, are there any other objects that belong in this category? How about Earth, which is the next biggest object? Of course; no question that we belong with the big kids. Venus is just a bit smaller, so sure, Venus. And so on. (This sketch is not historical, by the way. The planets [wanderers] for the Greeks were those celestial objects that moved against the background of fixed stars.) So how far down do you go with this category? Bear in mind that the objects under consideration range all the way down to dust particles and, beyond those, to random molecules and atoms. You have to stop somewhere, and where you stop depends entirely upon how you intend to use this category -- to identify objects that arose through a common process, for example.

When the definition of "planet" began to seem more porous than was useful -- meaning that it was loose enough that it could arguably apply to some objects that it was inconvenient in other ways to think of in the same category, like the asteroid Ceres or the faraway object 2003 UB313 -- it was time to reconsider the definition. That is all that the International Astronomical Union has done. The discussion of a new definition revolved (get it? revolved?) around the question of what the purpose of the definition was -- what objects and kinds of object do we wish to distinguish by this term "planet" and why? What kind of definition, incorporating what sorts of criteria, is meaningful for our work? In the end, by a large margin, a new definition that happens to exclude Pluto was adopted. Pluto is now the first citizen of a new category called "dwarf planet."

My newspaper tells me that this is a big deal in part because so many people have memorized the list of planets and now will apparently have to devote considerable processing power to remembering to delete the last one. Also, solar system mobiles will have to be redesigned. Textbooks have to be rewritten (ignoring the fact that textbooks are "rewritten" whether they need it or not, to keep publishers' revenues up). None of this explains page-one treatment.

There were questions about Pluto almost from the day Clyde Tombaugh discovered it in 1930. It didn't bother Tombaugh, it seems. His widow quotes him as having said before he died in 1997, "It's there. Whatever it is. It is there." That's the simple fact; the rest is just name-calling.

Robert McHenry is Former Editor in Chief, the Encyclopædia Britannica, and author of How to Know (Booklocker.com, 2004). He wrote recently about Ahmadinejad's blog. He is a TCS Daily Contributing Editor.

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