 The difference between aesthetic effects and the mechanisms that realise them is an important one. Take the shimmering quality we find in impressionist art. It is said that this depends on the fact that the visual system involves two separate processing streams and that certain colour combinations are processed exclusively by one of them (Livingstone 2002). Should this fact, if it is one, be regarded as an aesthetic fact? Arguably no. What matters aesthetically is the experienced shimmering quality; a world in which just that shimmering quality was produced by some other mechanism of perceptual processing would not be aesthetically different from our world, at least in this respect. The same applies to scientific studies of the stimuli themselves; chemical analysis of an ancient pigment might show that it was a hard medium to work in, and that would be relevant to understanding the works of art produced by its means. But what matters is the difficulty of the medium; discovering its chemical formula simply provides evidence for that.