He [Jasper Johns] and I were each other's first serious critics. Actually he was the first painter I ever shared ideas with, or had discussions with about painting. No, not the first. Cy Twombly was the first. But Cy and I were not critical. I did my work and he did his. Cy's direction was always so personal that you could only discuss it after the fact. But Jasper and I literally traded ideas. He would say, 'I've got a terrific idea for you' and then I'd have to find one for him.

As quoted in Lives of the great twentieth century artists, Edward Lucie-Smith, London 1986, p. 31