Someone comes to you and says:
Long have I pondered the meaning of the word "poetry", and at last I've found what seems to me a satisfactory definition: "Poetry is literature written in verse."
Just because there's a word "poetry" doesn't mean that it has a meaning, floating out there in the void, which you can discover by finding the right definition.
It feels that way, but it is not so.
Wondering how to define a word means you're looking at the problem the wrong way—searching for the mysterious essence of what is, in fact, a communication signal.
Now, there is a real challenge which a rationalist may legitimately attack, but the challenge is not to find a satisfactory definition of a word… The challenge is figuring out which things are similar to each other—which things are clustered together—and sometimes, which things have a common cause.
If you define "eluctromugnetism" to include lightning, include compasses, exclude light, and include Mesmer's "animal magnetism" (what we now call hypnosis), then you will have some trouble asking "How does electromugnetism work?" You have lumped together things which do not belong together, and excluded others that would be needed to complete a set.
We could say that electromugnetism is a wrong word, a boundary in thingspace that loops around and swerves through the clusters, a cut that fails to carve reality along its natural joints.
Figuring where to cut reality in order to carve along the joints—this is the problem worthy of a rationalist. It is what people should be trying to do, when they set out in search of the floating essence of a word.
What is "poetry"? But there is no essence of the word, floating in the void.
Perhaps you come to me with a long list of the things that you call "poetry" and "not poetry":
A punch in the nose: Not poetry.
Bible Psalms: Poetry.
A flower: Not poetry.
Paradise Lost: Poetry.
A cross floating in urine: Not poetry.
The Odyssey: Poetry.
And you say to me: "It feels intuitive to me to draw this boundary, but I don't know why—can you find me an intension that matches this extension? Can you give me a simple description of this boundary?"
But of course my definition of poetry is not the real point. The real point is that you could well dispute either the intension or the extension of my definition.
(You could dispute) my intension, my attempt to draw a curve through the data points. You would say, "Your equation may roughly fit those points, but it is not the true generating distribution."
Or you could dispute my extension by saying, "Some of these things do belong together—I can see what you're getting at… Here, the presumption is that there is indeed an underlying curve that generates this apparent list of similar and dissimilar things—that there is a rhyme and reason, even though you haven't said yet where it comes from—but I have unwittingly lost the rhythm and included some data points from a different generator.