If Pluto's Not a Planet, Is Ketchup a Vegetable, a Fetus a Person, or Same-Sex Marriage an Oxymoron? Natural Kinds in Science and Law

By MICHAEL C. DORF

Monday, Aug. 28, 2006

Last week the International Astronomical Union adopted a scientific definition of the term "planet" that leaves Pluto out in the cold. Demoted from its exalted place in our solar system, Pluto now joins a handful of other small distant objects to be denoted "planetary dwarves." (No, there aren't seven of them, and please, no jokes about my own name.)

The astronomers' decision was apparently difficult. Before the new definition was adopted, a pro-Pluto faction had proposed an alternative that would have accorded planetary status to any object orbiting the sun with sufficient gravity to pull itself into the shape of a sphere. That definition would have saved Pluto, but at the cost of defining dozens of additional objects, including some large asteroids, as planets. No doubt, millions of schoolchildren breathed a sigh of relief that they would not be required to learn the names of all these celestial spheres.

Meanwhile, those of us who are neither astronomers nor schoolchildren may find the Plutonic controversy, at best, amusing. Why should scientists care whether anyone attaches the label "planet" to Pluto or any other object? As Shakespeare might have said, a planet by any other name would go 'round the sun.

Indeed, the focus on labels seems especially odd for modern scientists, for it appears to mistake an arbitrary (if useful) category constructed by human beings, for a "natural kind"--a grouping with a sharp boundary determined by nature itself. Moreover, as I shall explain, philosophers debate whether the concept of "natural kind" itself makes any sense.

Yet, as I shall also explain, in life and especially in law, we frequently act as though the definitions we give to various words--words like "vegetable," "person," and "marriage"--are more than matters of mere convention. Can we make sense of this practice without resorting to the contested notion of natural kinds?

From Plato to Pluto

Although the term "natural kind" first appears in modern philosophy, it has roots in the works of the ancient Greeks. Plato believed that the world we experience is but an imperfect manifestation of an ideal world, filled with ideal "forms." The chair on which you sit is a material approximation of the ideal form of chair, the cup from which you drink an approximation of the ideal cup, and so forth.

Plato famously illustrated his conception of an ideal world in the Allegory of the Cave (in Book VII of The Republic), in which people seated in a cave can only view shadows on the wall, projected by lighted objects behind them. They understandably mistake the shadows for real things. These troglodytes, in Plato's allegory, represent all of us, able to see only shadows of the truth, which we mistake for reality.

The modern notion of "natural kinds" differs from Plato's forms in two important respects. First, modern philosophers do not typically share Plato's quasi-mystical conception of the forms, which, for him, appear to have a reality beyond their conceptual content. Second, no one today contends that every category is a natural kind. Chairs, cups, notebook computers, and other human artifacts are recognized as just that: artificial creations of human beings, not natural kinds. Philosophers, accordingly, do not debate whether a bean bag chair is "really" a chair, except to the extent that they aim to clarify how English speakers in a particular community use the word "chair."

Natural Kinds in Modern Science

But some scientists and philosophers continue to believe that other categories are natural kinds. A species is an arguable example. If species is defined as something like "a group of organisms that can interbreed with one another, but not with other organisms," then the members of a species could be thought to form a natural kind: whether they form such groups is a fact about nature, it would seem, not about our language.

However, the definition of a naturally occurring phenomenon is not itself a naturally occurring phenomenon. Horses and donkeys are usually considered separate species, even though they can interbreed to create mules. Some biologists thus modify the definition of "species" to require that interbreeding be capable of producing fertile offspring. (Mules are sterile.) Yet that addition feels ad hoc, proposed only to preserve the intuition that horses and donkeys should be grouped separately despite their ability to interbreed.

Elementary particles like electrons, protons and neutrons may be the best candidates for natural-kind-hood. After all, these are elementary particles; everything is made of them and they, in turn, cannot be broken down any further.

Except that it turns out that some of them can. Just as the ancients' elements of earth, wind, fire, and water turned out to be compound phenomena, so protons and neutrons (but not, so far as we know yet, electrons), are made of still smaller particles called quarks. And since physicists began smashing particles into each other at high speeds about half a century ago, new particles, such as muons, positrons and neutrinos, have been discovered.

Should we consider electrons and positrons part of one natural kind, because they are identical except for bearing the opposite charge? Or is charge crucial to the kind, so that electrons form one natural kind, while positrons form another? The question has the same feel as the question whether the ability to produce fertile offspring is crucial to species-hood: The more one thinks about the question, the more it seems to be asking about how we use words, rather than about things in the real world.

Accordingly, we might well be skeptical about the existence of any natural kinds, even in the sciences. But what about in the law?

Natural Kinds in the Law? Defining Vegetable

If animal species and particles--which exist in the world independent of our efforts to define them--do not constitute natural kinds, then one would think that, surely, legal terms such as "corporation," "property interest" and "cruel and unusual punishment" do not constitute natural kinds either. After all, the latter terms are created by people to govern their own affairs.

Nonetheless, debates about what the law is, or should be, often proceed as though our definitions of words aimed to capture phenomena that already existed in the real world.

For example, in the first year of the Reagan Administration, the Department of Agriculture proposed rules that would permit states to count ketchup as a vegetable in calculating what qualified as a reimbursable school lunch. Nutritionists and many Democrats cried foul: Ketchup, everyone knows, is a condiment, not a vegetable.

But is it? Of course, ketchup is not itself a vegetable, but then neither are, say, string beans with sliced almonds lightly sautéed in olive oil. Such prepared foods are, however, made from vegetables. (All right, a tomato is actually a fruit, not a vegetable, but surely that was not the objection to the Reagan plan.)

Nor is the term "condiment" self-defining. We might say condiments are not foods themselves, but foodstuffs placed on other foods. Yet plenty of things we would call vegetables can be placed on other foods without losing their vegetable-ness. Lettuce and tomatoes have the same nutritional value whether eaten by themselves or in a sandwich as a topping.

So if neither "vegetable" nor "condiment" is a natural kind, were critics wrong to assail Reagan's Agriculture Department for its proposed redefinition of ketchup? Certainly not. The objection was not that ketchup isn't "really" a vegetable. The objection was that through the redefinition, the Department sought to permit states to avoid responsibility for providing healthy meals to kids.

Defining "Person"

Turning from the farcical to the very serious, consider struggles between pro-choice and pro-life activists over how to define "person." Pro-lifers typically say that from the moment of conception, a human zygote-becoming-embryo-becoming-fetus is a person, with the same rights as other persons. Although some pro-choicers (including yours truly) think that the abortion right can be justified even assuming that fetuses are persons, many treat the status of the fetus as crucial, and deny its personhood. The Supreme Court itself made this move in Roe v. Wade. The Court thought that acknowledging fetal personhood would mean the "collapse" of the case for an abortion right, and then denied that fetuses are persons under the Constitution.

The Justices in Roe were interested in whether fetuses should count as persons, given the language and history of the Fourteenth Amendment. But in public debate about abortion, the issue is not what the Constitution says about fetuses; the issue is whether fetuses really are persons. One senses on both sides a reliance on natural kinds.

If we think that natural kinds do not exist, does that mean that the debate about personhood is nonsensical? Not necessarily. We might best understand the disputants as saying something like this: "Fetuses have (or don't have) all of the characteristics of persons that make persons worthy of protection against certain kinds of wrongs, and abortion therefore is (or is not) a wrong to a being that should have the same legal status as a person." Whether one takes the further step, and says that a fetus is (or is not) a person, is not important to the argument.

What is "Marriage?"

Nonetheless, in some public debates it appears that people really are just quibbling over words or, worse, giving normative force to habitual ways of speaking. The controversy over same-sex marriage may fall into this category.

Some opponents of same-sex marriage contend that concrete harms will result from extending legal recognition to same-sex couples. They contend that it will weaken respect for the institution of marriage generally, or they make claims about adverse effects on children. To my mind, these arguments are quite unpersuasive, but at least they take the form of arguments. They accept that we could change the legal definition of marriage, and then offer reasons for why we should not.

But a large number of people oppose same-sex marriage on what appear to be definitional grounds. Marriage is an institution between one man and one woman, they say. Thus, by definition, people of the same sex cannot marry. (Federal law and many state laws do, in fact, define marriage exactly this way, but those who make the definitional argument do not rely on these laws; they propose that there is a deeper, universal definition of marriage that would hold true regardless of what the law said.)

Can we reconstruct this claim along the lines that we reconstructed the claims about the personhood of the fetus? Perhaps. Maybe when people say that marriage must be defined as an institution between one man and one woman, they mean something like the following: "The relationship between one man and one woman that has traditionally been recognized as marriage has certain characteristics, such as its essential role in raising children, that would not be served, or not be served as well, by extending the definition to encompass same-sex couples."

If that is what same-sex marriage opponents mean, then no harm results from the use of the definitional point as a kind of shorthand, so long as they are prepared to make actual arguments on the merits. They must be willing, in other words, to accept same-sex marriage if their assumptions about same-sex couples, child-rearing, and other issues prove to be false.

On this, as on so many issues, much depends on political leadership. National Republican leaders have tended to play to their religious conservative base in opposing same-sex marriage, but Democrats have not been much better. For example, in the 2004 Presidential election, John Kerry repeatedly stated that he opposed same-sex marriage because of his personal belief that "marriage is between a man and a woman."

When political leaders substitute tautologies for analysis, it's not surprising that ordinary citizens do the same. Indeed, as the astronomers' heated debate about Pluto shows, even rocket scientists sometimes have a hard time getting past mere words.


Michael C. Dorf is the Michael I. Sovern Professor of Law at Columbia University. He is the author of No Litmus Test: Law and Politics in the Twenty-First Century.

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