The one comes to you and says:
Long have I pondered the meaning of the word "Art", and at last I've found what seems to me a satisfactory definition: "Art is that which is designed for the purpose of creating a reaction in an audience."
Just because there's a word "art" 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 real challenge can be played as a single-player game, without speaking aloud. 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. (This example is historically plausible; Mesmer came before Faraday.)
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.
And make no mistake: it is a scientific challenge to realize that you need a single word to describe breathing and fire. So do not think to consult the dictionary editors, for that is not their job.
What is "art"? 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 "art" and "not art":
The Little Fugue in G Minor: Art.
A punch in the nose: Not art.
Escher's Relativity: Art.
A flower: Not art.
The Python programming language: Art.
A cross floating in urine: Not art.
Jack Vance's Tschai novels: Art.
Modern Art: Not art.
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?"
So I reply: "I think it has to do with admiration of craftsmanship: work going in and wonder coming out. What the included items have in common is the similar aesthetic emotions that they inspire, and the deliberate human effort that went into them with the intent of producing such an emotion."
Is this helpful, or is it just cheating at Taboo? I would argue that the list of which human emotions are or are not aesthetic is far more compact than the list of everything that is or isn't art. You might be able to see those emotions lighting up an fMRI scan—I say this by way of emphasizing that emotions are not ethereal.
But of course my definition of art 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 say, "Aesthetic emotion is not what these things have in common; what they have in common is an intent to inspire any complex emotion for the sake of inspiring it." That would be disputing 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—but the Python language shouldn't be on the list, and Modern Art should be." (This would mark you as a gullible philistine, but you could argue it.) 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.
Long before you know what it is that electricity and magnetism have in common, you might still suspect—based on surface appearances—that "animal magnetism" does not belong on the list.
Once upon a time it was thought that the word "fish" included dolphins. Now you could play the oh-so-clever arguer, and say, "The list: {Salmon, guppies, sharks, dolphins, trout} is just a list—you can't say that a list is wrong. I can prove in set theory that this list exists. So my definition of fish, which is simply this extensional list, cannot possibly be 'wrong' as you claim."
Or you could stop playing nitwit games and admit that dolphins don't belong on the fish list.
You come up with a list of things that feel similar, and take a guess at why this is so. But when you finally discover what they really have in common, it may turn out that your guess was wrong. It may even turn out that your list was wrong.
You cannot hide behind a comforting shield of correct-by-definition. Both extensional definitions and intensional definitions can be wrong, can fail to carve reality at the joints.
Categorizing is a guessing endeavor, in which you can make mistakes; so it's wise to be able to admit, from a theoretical standpoint, that your definition-guesses can be "mistaken".
I have no idea what you mean by modern art. "Modern" referring to the time of creation or to a loose collection of specific artistic movements thriving mostly in the first half of the 20th century? Neither makes much sense to me when declared non-art; the first seems to include everything contemporary while the second includes, for example, Picasso or Virginia Woolf.
Modern-art is a specific type of art that was dominant between about 1860 to 1970'ish. Famous modern-artists are Picasso, and Van Goh, among others. It tends to be quirky and weird, but much more structured than simply splashing buckets of paint on a canvass.
A lot of people consider the vast majority of it to be trash. It's telling that it can be difficult to distinguish between modern art created by truly talented painters like Picasso and a hack who couldn't paint a simple bowl of fruit.
But then, art is fuzzy and subjective, so modern-art is not entirely dead.