<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> <P> Canadian songwriter, social activist and environmentalist Bruce Cockburn poses the question in the chorus of his 1989 song "If a Tree Falls" and frames it with his lyrics as a pressing question regarding the cause and effect of deforestation . </P>

If a dead tree falls in the woods does it make a sound