So that's a point against it existing, but maybe you find the fallen tree later and ask if it was loud when it fell. Most people would agree a tree works the exact same way watched or not, though. There's different justifications why. Some people would say ontological momentum; I'd point to Occam's razor, which can be mathematically derived from Solomonoff calculus, and the laws of physics we have which can fit on a pamphlet and are supposed to apply anywhere at any time.