Art 101: "What is the reasoning of the artist signified by this artifact?"
Broadly speaking, this question may have both a superficial answer -- e.g., "He wanted to make a painting of a vase of flowers" -- and a more substantial answer -- e.g., "He wanted to show the fragility and fleetingness of beauty."
At some point, artifacts started showing up for which the superficial answer was, "Who knows?" Many art critics were delighted, since they could roll up their sleeves and invent whatever "more substantial" answers they felt like.
Artists were delighted too, of course. Not only is portrait painting hard to do well, but once artifacts that signify no particular evident reasoning were accepted as works of art, the artists themselves were free to provide the reasoning themselves. A cipher came to "mean" whatever the artist said it meant, because it certainly didn't mean anything else.
This approach to art offers artists two benefits. First, the actual making of the artifact is no big deal. It doesn't really matter what something intended to occupy space next to a textual description of what it means looks like. The artist can't fail at creating his artifact.
Second, the artist is more important than his artifacts. Critics and writers need to listen to the artist to understand what a piece means. And if people need to listen to the artist, then the artist must really be somebody in the world of ideas.
If I knew what I was talking about, I might distinguish between answering the question, "What is the reasoning of the artist signified by this artifact?" with, "Who knows?," and answering it with, "Nothing in particular." Various schools of abstract art seek to produce artifacts that don't signify anything much beyond, "Say, this is sort of interesting to look at," or, "This is what happens when I do that." Then, rather than an impenetrable paragraph of text, you get something like, "Composition #12."
Eh. It's a living.
No, actually it is, or can be, a perfectly legitimate process of human exploration. "What does this do?" is a fundamental question of natural philosophy.
Note, by the way, that -- despite my editorializing -- this post's titular question really does seek an objective answer. Evaluating an artifact -- determining its artistic or social value -- comes later.