<P> Can we assume the unobserved world functions the same as the observed world?--e.g., "does observation affect outcome?" A similar question does not involve whether or not an unobserved event occurs predictably, like it occurs when it is observed . 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 as follows, "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 . It may then be recognized that the unobserved event was absolutely identical to an event which did not occur at all ." (this apparent quote has no attribution or reference and none can be found online with reasonable effort). Of course, the fact that the tree is known to have changed state from' upright' to' fallen' implies that the event must be observed to ask the question at all--even if only by the supposed deaf onlooker . The British philosopher of science Roy Bhaskar, credited with developing critical realism has argued, in apparent reference to this riddle, that: </P> <P> If men ceased to exist sound would continue to travel and heavy bodies to fall to the earth in exactly the same way, though ex hypothesi there would be no - one to know it </P> <P> This existence of an unobserved real is integral to Bhaskar's ontology, which contends (in opposition to the various strains of positivism which have dominated both natural and social science in the twentieth century) that' real structures exist independently of and are often out of phase with the actual patterns of events' . In social science, this has made his approach popular amongst contemporary Marxists--notably Alex Callinicos--who postulate the existence of real social forces and structures which might not always be observable . </P> <P> What is the difference between what something is, and how it appears?--e.g., "sound is the variation of pressure that propagates through matter as a wave" Perhaps the most important topic the riddle offers is the division between perception of an object and how an object really is . If a tree exists outside of perception then there is no way for us to know that the tree exists . So then, what do we mean by' existence', what is the difference between perception and reality? Also, people may also say, if the tree exists outside of perception (as common sense would dictate), then it will produce sound waves . However, these sound waves will not actually sound like anything . Sound as it is mechanically understood will occur, but sound as it is understood by sensation will not occur . So then, how is it known that' sound as it is mechanically understood' will occur if that sound is not perceived? </P>

Koan if a tree falls in the forest