Why does Weitz say that art’s “openness” makes a definition impossible?:

What Morris Weitz means is that Art continually evolves, mixing up the categories in art and moving them around so much that it disrupts other categories that he believes it is impossible to define art in such a way he believes all other word can be directly defined. Because of the fact that art has no boundaries and is considered a practice make the definition of art indefinable. There are also just so many varieties, parts, sections and exceptions in art for it to be described in a coherent way. As art changes throughout evolution, the definition also changes along with it. Art can be looked at in many ways, as what Morris Weitz comments about L. Wittgenstein's essay, art could be looked at as a game. 

Weitz mentions how although art has many categories, such as "'tragedy,' 'comedy,' 'painting,' 'opra,' etc." Only thing is that art has to fit perfectly into that category description, other wise, there would have to be a new category made in art. You can compare art to a family for example, as Weitz tries to explain. For every family member, one person can have brown hair, blue eyes, and freckles. This family member might look similar to another family member but have different resemblances. For example, family member number one might have straight hair and family member number two might have curly hair. This is the way Morris Weitz thinks of how words are defined. He believes that words can seem like they mean the same thing but have a distinct definition that makes the words different from each other. And that is why he believes art is indefinable.

Shouldn't there be certain principles that the definition of art should follow? If there are, what are they?