If something cannot be defined, can it exist?

Usually, when we use the word "defined," it's about words. We might ask, for example, whether the word "chair" can be defined. If a definition is supposed to rule in exactly the things that are chairs and rule out exactly the things that aren't, then I doubt that "chair" can be defined. We can, of course, say things about what typical chairs are like. But any supposed strict definition we offer will, I'm willing to bet, have various counterexamples. Typical chairs are for sitting on. But benches aren't chairs, and we often sit on benches. And some chairs---certain works of art, perhaps---aren't actually meant for sitting on. And yet there are chairs. In fact, I'm sitting on one as I type this.

However, there's another use of the word "defined" that isn't about words but about things. When we ask for definitions in this sense, we're asking for an account of the nature of the thing itself. For example: it's of the nature of an electron that it has negative charge and spin one-half. Or so we might say. This isn't a point about the meaning of the word "electron." It's a point about what the things we refer to by that word are like "by nature." We might say that electrons both exist and have a nature. If we carve the world at its joints, as its sometimes put, electrons would be one of the "natural kinds" we included in our account.

Can something exist that doesn't have a nature in this sense?

How about chairs? There really are chairs. I'm still sitting on one. But this chunk of wood, glue and screws that's supporting my weight doesn't seem to have a nature in anything like the way that electrons do. That certain hunks of matter count as chairs is a matter of convention, dependent on human purposes and practices. That doesn't seem to be so in the case of electrons. They seem to be instances of a real kind and not just a reflection of our conventions and intentions.

Not everyone will agree. The view that there aren't really any real kinds has a long history, going back at least to some strains of Hindu and Buddhist thought, having been re-argued by Medieval nominalists, defended more recently by Nelson Goodman and having become a platitude of post-modern thought. My instincts are more with the other side in this dispute, but even if the nominalist and their fellow-travelers are right, indeed especially if they're right, there's much that exists in the ordinary way that my chair does even if the words we use to pick it out don't have strict definitions, and even if things don't even have natures. After all: that certain pieces of paper count as money is clearly not written into nature itself, but don't let anyone use that as an excuse not to pay you your salary.

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