Andy Warhol’s Brillo Boxes is a stack of boxes that are visually indistinguishable from the product packaging of an actual brand of scouring pads.
Conclusion
It's wrong to say "appearance alone entirely determines whether something is art". (translate: something besides appearance is involved in determining whether something is art)
Evidence
Warhol's BB's appears the same as a normal stack of BB's would. However, Warhol's is considered art, whereas a normal stack is not art.
Evaluate
The author is playing off an implicit contrapositive idea:
Since we learn that Warhol's scouring pads are art, but a normal stack wouldn't be, it triggers the contrapositive and tells us that "appearance can't be all that matters".
The author is refuting a generalization. She's shooting down the idea that . All she needs to do to shoot it down is present a counterexample, which she does: we see, with Brillo Boxes, a case in which the appearance is the same but the whether-or-not it's art is different.
Goal
Probably look for language like .