It's possible to define what art is?

The wider question is what is the point of defining anything. Definitions really do not help us know anything precise about anything, they just establish some rough and ready parameters around the meaning of the term. Provided we grasp that, then definitions do no harm, but their application to try to restrict what we can say and do with terms causes more harm than good. Often we can hit the term right out of the ballpark without losing it, and this is very much the case with attempted definitions of something as complex as art.

A definition of art is useful to someone learning the language, but otherwise has no useful purpose.

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