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If a tree falls in a forest

A fallen tree in a forest

"If a tree falls in a forest and no one is around to hear it, does it make a sound?" is a philosophical thought experiment that raises questions regarding observation and perception.



Philosopher George Berkeley, in his work, A Treatise Concerning the Principles of Human Knowledge (1710), proposes, "But, say you, surely there is nothing easier than for me to imagine trees, for instance, in a park [...] and nobody by to perceive them.[1] [...] The objects of sense exist only when they are perceived; the trees therefore are in the garden [...] no longer than while there is somebody by to perceive them."[2] (It is worth noting that the quote from section 45 is arguably a statement of an objection to Berkeley's view, and not a proclamation of it.) Nevertheless, Berkeley never actually wrote about the question.[3]

Some years later, a similar question is posed. It is unknown whether the source of this question is Berkeley or not (Indian Hindu saint of 8th century Sri Adisankara raised similar question "If no one is seeing sky whether it will be blue?"). In June 1883 in the magazine The Chautauquan, the question was put, "If a tree were to fall on an island where there were no human beings would there be any sound?" They then went on to answer the query with, "No. Sound is the sensation excited in the ear when the air or other medium is set in motion."[4] This seems to imply that the question is posed not from a philosophical viewpoint, but from a purely scientific one. The magazine Scientific American corroborated the technical aspect of this question, while leaving out the philosophic side, a year later when they asked the question slightly reworded, "If a tree were to fall on an uninhabited island, would there be any sound?" And gave a more technical answer, "Sound is vibration, transmitted to our senses through the mechanism of the ear, and recognized as sound only at our nerve centers. The falling of the tree or any other disturbance will produce vibration of the air. If there be no ears to hear, there will be no sound."[5]

Albert Einstein is reported to have asked his fellow physicist and friend Niels Bohr, one of the founding fathers of quantum mechanics, whether he realistically believed that 'the moon does not exist if nobody is looking at it.' To this Bohr replied that however hard he (Einstein) may try, he would not be able to prove that it does, thus giving the entire riddle the status of a kind of an infallible conjecture—one that cannot be either proved or disproved.

The current phrasing appears to have originated in the 1910 book Physics by Charles Riborg Mann and George Ransom Twiss. The question "When a tree falls in a lonely forest, and no animal is near by to hear it, does it make a sound? Why?" is posed along with many other questions to quiz readers on the contents of the chapter, and as such, is posed from a purely physical point of view.[6]


The possibility of unperceived existenceEdit

Can something exist without being perceived? – e.g. "is sound only sound if a person hears it?" The most immediate philosophical topic that the riddle introduces involves the existence of the tree (and the sound it produces) outside of human perception. If no one is around to see, hear, touch or smell the tree, how could it be said to exist? What is it to say that it exists when such an existence is unknown? Of course, from a scientific viewpoint, it exists.[7] It is human beings that are able to perceive it.[7] George Berkeley in the 18th century developed subjective idealism, a metaphysical theory to respond to these questions, coined famously as "to be is to be perceived". Today meta-physicists are split. According to substance theory, a substance is distinct from its properties, while according to bundle theory, an object is merely its sense data. The definition of sound, simplified, is a hearable noise. The tree will make a sound, even if nobody heard it. The definition states that sound is a hearable noise. So the tree could have been heard, though nobody was around to do so.

Knowledge of the unobserved worldEdit

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:

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[8]

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'.[9] 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.[10][11][12]

The dissimilarity between sensation and realityEdit

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?

In popular cultureEdit

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.

Washington-state-based wilderness conservatory Northwest Trek used a shortened form of the quote in its mid-1970s television advertisement, as such: "There is no sound unless someone is there to see it or hear it. Experience it at Northwest Trek."

The quote also forms the bridge of the protagonist's solo number "Waving Through A Window" in the musical Dear Evan Hansen, in line with the tree motif essential to the plot. The song itself discusses a feeling of isolation through fear of failing in social interactions, as a part of the main character's social anxiety disorder.

See alsoEdit


  1. ^ A Treatise Concerning the Principles of Human Knowledge, 1734. section 23.
  2. ^ A Treatise Concerning the Principles of Human Knowledge, 1734. section 45.
  3. ^ Whether Berkeley thought objects continued to exist when unperceived by finite minds, and if so, in what manner, is the subject of serious debate among Berkeley scholars.
  4. ^ The Chautauquan, June 1883, Volume 3, Issue 9, p. 543
  5. ^ Scientific American, April 5, 1884, pg 218.
  6. ^ Mann, Charles Riborg and George Ransom Twiss. Physics. Scott, Foresman and Co., 1910, p. 235.
  7. ^ a b "What is Philosophy? – Analysis", Plymouth State University, Philosophy Department
  8. ^ Bhaskar, R. (2008 [1975]), A Realist Theory of Science, London: Verso, p. 21.
  9. ^ Bhaskar, R. (2008[1975]), A Realist Theory of Science, London: Verso, p. 13.
  10. ^ Marsh, D. (2002), "Marxism", in Marsh D. Stoker, G. (Eds.), Theory and Methods in Political Science, Basingstoke: Palgrave Macmillan, p. 159.
  11. ^ Marsh, D, & Furlong, P. (2002), “Ontology and Epistemology in Political Science”, in Marsh D. Stoker, G. (Eds.), Theory and Methods in Political Science, Basingstoke: Palgrave Macmillan, p. 31.
  12. ^ Callinicos, A. (2006), The Resources of Critique, Cambridge: Polity, pp. 155–158.