Full Idea
What in the end makes the difference between a Brillo box and a work of art consisting of a Brillo box is a certain theory of art. It is the theory that takes it up into the world of art, and keeps it from collapsing into the real object which it is.
Gist of Idea
An ordinary object can be a work of art, but only if some theory of art supports it
Source
Arthur C. Danto (The Artworld [1964], p.581), quoted by Sondra Bacharach - Arthur C. Danto
A Reaction
It is hard to describe Duchamp's original claim that the urinal was an artwork as a 'theory'. It is a mere rebellious assertion.
Book Reference
'Key Thinkers in Aesthetics', ed/tr. Giovannelli,Alessandro [Continuum 2012], p.191