The better question is, how do we know whether a tree or person is or is not even heard or not heard if we do not know everything, how can we know that something or someone is not heard or will never be heard? If it is not heard, can be seen, smelt, felt, touched, sensed, and does that matter? I may not hear you, but can I see you? I may not hear the tree fall, but I may see a fallen tree someday.