Clement Greenberg talked about the ideas or possibilities of painting in his - I think -, 'After Abstract Expressionism' article, and he allows a blank canvas to be an idea for a painting. It might not be a good idea, but it's certainly valid. Yves Klein did the empty gallery. He sold 'air', and that was a conceptualized art, I guess.

p. 122 - 'Questions to Stella and Judd' - September 1966