Heil, John - Philosophy of mind: a contemporary introduction

Does a tree falling in the forest make a sound when no one is around to hear it? The question is familiar to every undergraduate. One response is that of course the tree makes a sound—why shouldn’t it? The tree makes a sound whether anyone is on hand to hear it or not. And, in any case, even if there are no people about, there are squirrels, birds, or at least bugs that must hear it crashing down.

Consider a more measured response, versions of which have percolated down through successive generations of undergraduates. The tree’s falling creates sound waves that radiate outwards in a spherical pattern. If these sound waves are intercepted by a human ear (or maybe—although this is more controversial—the ear of some non-human sentient creature) they are heard as a crashing noise. If the sound waves go undetected, they eventually peter out. Whether an unobserved falling tree makes a sound, then, depends on what you mean by sound. If you mean “heard noise,” then (squirrels and birds aside) the tree falls silently. If, in contrast, you mean something like “distinctive spherical pattern of impact waves in the air,” then, yes, the tree’s falling does make a sound.