Pluto – Courtesy of NASA/Johns Hopkins APL

I love words. I also love science. And, because I love both, I pay attention to the interesting places where the words of science and the words of society do not quite match. Scientific terms need to reflect a series of characteristics shared by the things they describe, and we need to know what those characteristics are. Scientists need to be able to look at new astronomical bodies and categorize them, so the “I know it when I see it” approach that works for most of the rest of us most of the time just isn’t good enough in science. From that perspective, it makes sense to update the definitions of things to reflect our growing knowledge.

The word “planet” is one such example. Yes, it was infamously revised (scientifically) to exclude Pluto as a planet. There were lots of good reasons for this, not the least of which being that scientific language, unlike regular language, needs to limit its fuzzy edges. And people, including me, were sad to see Pluto dropped from the A-list in our solar system. Scientifically, it makes sense for now—but even in science, language changes.

Last week I was traveling and exploring places few people ever go. At the same time, the New Horizons spacecraft made its closest approach to Pluto, visiting it more closely than we ever have before. Over the coming weeks and months we will learn more about that tantalizing little world than we have ever known. Maybe Pluto will shift categories yet again.

Pluto is a good example of the fuzziness of words, because the scientific redefinition of the word “planet” exposed the fuzzy definition we had been using for a long time. It’s not that “planet” is unique, either. In the rest of society, words are not defined by hard and fast categories, because language is fundamentally democratic.

What about dictionaries, you ask? Dictionaries describe words; they don’t prescribe meaning. If most people use a word a particular way, that is a meaning of that word. If other people use it differently, the word has a second meaning. And so on. And there are lots of places where the dictionary or scientific definition of a word does not encompass everything it can mean. For example, have you ever witnessed an argument about whether tomatoes are fruits or vegetables? It depends on your definition. Colloquially, they are vegetables. Get a little more specific and they are fruit. Get even more specific and they are vegetables again—along with all fruits, and grains besides. But the whole argument misunderstands the point; tomatoes are not inherently one word or another, they just are; we decide what to call them, and when, and why.

Let’s try again—how about the word “theory?” In scientific terms, a theory is a general explanation of some phenomena that is deeply and broadly supported by the evidence. In colloquial terms, a theory may be nothing more than an educated guess. This is why many people arguing about the “theory of evolution” consistently and fundamentally misunderstand the subject. The edges of the word “theory” are fuzzy.

And let’s look at one more example, something you might not think has fuzzy edges (except in reality): a dog. Picture a dog in your mind. Dogs have paws, long noses, ears, four legs, tails. What about a dog with only three legs? Is it still a dog? What about dogs without tails? Still dogs? What about a cross between a dog and a wolf? It meets all the qualifications to be a dog, except something weird happens there because it meets the qualifications for a wolf as well. So which is it? Or is it both? The edges are fuzzy.

A moment ago you might have tried to think up a definition of “dog” that excludes my edge cases, maybe something about DNA. If so, you’re not alone; the reaction of most people, when faced with a fuzzy edge, is to clarify it—to draw an arbitrary line and defend that line heartily. Tomatoes are fruit, end of story.

But must we? Language doesn’t actually work this way—everything has a fuzzy edge of you look close enough, and that’s a good thing. It is a deep reminder that words are imperfect stand-ins for reality. Scientific language can and must be precise, but the rest of the time we can have fun. We can have words that mean different things to different people. This is how language works. This is how language lives and grows.

So let me return to Pluto for a moment. Now we are closer to Pluto than we have ever been. New Horizons has taken hundreds of photos and collected amazing data about an alien world. It is a world that has sparked our imaginations, inspired us, captivated us, and all from a long cold orbit on the edge of our solar system. It is a tiny world on the fuzzy edge between our definitions, and between us and the rest of the universe. Is Pluto a planet? It doesn’t matter. The edges may be fuzzy, but today, Pluto is looking beautifully clear.