From
https://en.wikipedia.org/wiki/Fountain_(Duchamp):
> Fountain is a readymade sculpture by Marcel Duchamp in 1917, consisting of a porcelain urinal signed "R. Mutt". In April 1917, an ordinary piece of plumbing chosen by Duchamp was submitted for an exhibition of the Society of Independent Artists, the inaugural exhibition by the Society to be staged at The Grand Central Palace in New York. When explaining the purpose of his Readymade sculpture, Duchamp stated they are "everyday objects raised to the dignity of a work of art by the artist's act of choice."...
> The work is regarded by art historians and theorists of the avant-garde as a major landmark in 20th-century art.
So here's a major artist, saying that a urinal can be a work of art, needing only the artist's choice to make them so. And here's a major art movement agreeing that that "art" piece is a major landmark.
That doesn't quite claim "there's no such thing as beauty", but it pretty much says there's no such thing as art, other than an artist saying so. But then, who is an artist? Whoever says they are?
And if all it takes is the artist saying so, then a urinal is just as much art as the Mona Lisa.
But I think this does somewhat trace back to febeling's point. If nature is all there is, and nature is deterministic, then art is just what deterministic people do. It's not intrinsically different from non-art. And when people believe that, they act like what they do is just the same as the Mona Lisa, when we can look at it and tell that no, it's not.