Eighteenth-century European aesthetics was reasonably successful in providing an understanding of all art, including early abstract art, until the 1960s, when artists self-consciously rebelled against earlier notions of art.
Conclusion
There can be no complete theory of aesthetics.
Evidence
We thought we had a pretty successful theory that seemed complete, until the 1960s when rebellious artists managed to make beautiful stuff that didn't fit within earlier notions of art.
Evaluation
The author thinks that since these 1960s rebels made art that should be encompassed by a complete theory of aesthetics, but isn't encompassed by the 18th century aesthetic theory, then we'll just never have a complete theory.
We should probably be responding with something like,
Goal
Look for an answer naming the assumption that , or an answer raising the objection that there might one day be a theory that covers what the 18th century theory covered as well as the weird 60s stuff.