It's an old epistemological debate, older, actually, than the koan about the tree in the forest. (Epistemology is the investigation of what distinguishes justified belief from opinion.) Plato weighed in on it, and philosophers for two millennia afterward: What is beauty? Is it a measurable fact (Gottfried Leibniz), or merely an opinion (David Hume), or is it a little of each, colored by the immediate state of mind of the observer (Immanuel Kant)?