When it was discovered that the curtain itself was Parrhasius' painting, Zeuxis was forced to concede defeat, for while his work had managed to fool the eyes of birds, Parrhasius had deceived the eyes of an artist.
When it was discovered that the curtain itself was painted on the surface--the curtain was Parrhasius'painting, Zeuxis was forced to concede defeat.' In other words, while his work had managed to fool nature, i.e. the eyes of birds, Parrhasius' work had deceived the eyes of an artist.