 The anthropic principle suggests that the observer, just in its existence, may impose on the reality observed.However, most people, as well as scientists, assume that the observer doesn't change whether the tree-fall causes a sound or not, but this is an impossible claim to prove.However, many scientists would argue that a truly unobserved event is one which realises no effect (imparts no information) on any other (where 'other' might be e.g., human, sound-recorder or rock), it therefore can have no legacy in the present (or ongoing) wider physical universe.