Here's an excellent review of the philosophy of conceptual art. There are, perhaps, two principal principles expounded here:
1) Artworks acquire their status by virtue of their relations to the historical and social context, constituted by the practices and conventions of art, our artistic heritage, the intentions of artists, the expectations of the social class interested in art, etc. Art and non-art are not perceptually distinguishable, but a function of social relations.
2) Conceptual art performs a cognitive function rather than an aesthetic function.