If a tree falls in a forest
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. [...] 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." Nevertheless, Berkeley never actually wrote about the question.
Berkeley's example is referred to by William Fossett twenty years later in a consideration of the emergence of meaning: "[T]ease apart the threads [of the natural world] and the pattern vanishes. The design is in how the cloth-maker arranges the threads: this way and that, as fashion dictates. [...] To say something is meaningful is to say that that is how we arrange it so; how we comprehend it to be, and what is comprehended by you or I may not be by a cat, for example. If a tree falls in a park and there is no-one to hand, it is silent and invisible and nameless. And if we were to vanish, there would be no tree at all; any meaning would vanish along with us. Other than what the cats make of it all, of course."
Some years later, a similar question is posed. It is unknown whether the source of this question is Berkeley or not. 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." 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."
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.
The possibility of unperceived existence 
Can something exist without being perceived? - Quoted from Pamela Jackson - 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. It is human beings that are able to perceive it. 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-physicians 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 world 
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.". 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
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 .
The dissimilarity between sensation and reality 
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?
This riddle illustrates John Locke's famous distinction between primary and secondary qualities. This distinction outlines which qualities are axiomatically imbibed in an object, and which qualities are ascribed to the object. That is, a red thing is not really red (that is, "red" is a secondary quality), a sweet thing is not really sweet, a sound does not actually sound like anything, but a round object is round.
Buddhist perspectives 
from The Thirty-Seven Bodhisattva Practices by Ngulchu Thogme Zangpo,
"22 Appearances are one's own mind. From the beginning, mind's nature is free from the extremes of elaboration. Knowing this, not to engage the mind in subject-object duality is the bodhisattva's practice."
- "Two monks were arguing about the temple flag waving in the wind. One said, "The flag moves." The other said, "The wind moves." They argued back and forth but could not agree.
- The Sixth Ancestor said, "Gentlemen! It is not the wind that moves; it is not the flag that moves; it is your mind that moves." The two monks were struck with awe."
- - The Mumonkan Case 29, translation by Robert Aitken
One day the monks at Huang-mei monastery were instructed to write a stanza in order that their master Hung-jen might decide who would inherit the dharma throne. All of the monks assumed that a certain senior disciple Shen-hsui would win and thus didn't even bother writing stanzas. Shen-hsui assumed he would win too, and wrote the following stanza on the monastery wall:
Due to the danger surrounding him if he were to openly challenge the senior monk Shen-hsui, Hui-neng went out one night while it was completely dark and wrote the following stanza in secret:
- Bodhi originally has no tree.
- The bright mirror also has no stand.
- Fundamentally there is not a single thing.
- Where could dust arise?
Later, master Hung-jen saw this stanza and reaffirmed that he who had written it had indeed opened his mind's eye. Finding it was Hui-neng, master Hung-jen then gave Hui-neng the secret dharma teachings of their lineage, which Hui-neng says completely opened his mind, and gave the young Hui-neng the dharma robe and stick, symbolizing his status as Patriarch of the lineage. Hung-jen told Hui-neng that he must then flee the monastery for fear of Shen-hsui and his followers killing Hui-neng. Hui-neng did as he was told and then later returned from living with some hunters to become The Sixth Patriarch and pass on The Dharma in such forms as The Sutra Spoken by The Sixth Patriarch. Hui-neng also started the Sudden School which taught that Mind is realized instantaneously while Shen-hsui started the Gradual School which taught, similar to his stanza above, a sort of dust-wiping meditation. Zen today embodies Hui-neng's Sudden School style, particularly with Satori.
The ancient Hindu texts, the Vedas and the Puranas, have defined several theories of creation--one of them being the 'Drishti Srishti Vada' (Drishti=Sight, Srishti=Universe, Vada=Theory), which puts forth the argument that the universe, or that which is seen, arises only after the seer, and has no independent existence apart from the seer. The very name of the theory is derived from the order of precedence of sight and the seen--first there is sight ('Drishti') and then there is the seen ('Srishti').
"The mind is like akasa (space). Just as there are the objects in the akasa, so there are thoughts in the mind. The akasa is the counterpart of the mind and objects are of thought. One cannot hope to measure the universe and study the phenomena. It is impossible. For the objects are mental creations...The universe is only an object created by the mind and has its being in the mind." (Talks with Sri Ramana Maharshi, pg 478-479).
"The shrutis (a subdivision of the Vedas) and the sages say that the objects are only mental creations. They have no substantive being. Investigate the matter and ascertain the truth of the statement. The result will be the conclusion that the objective world is in the subjective consciousness."(TWSRM, pg 479).
See also 
- Object (philosophy)
- Observer effect (physics)
- Schrödinger's cat
- Subject-object problem
- Principle of locality
- A Treatise Concerning the Principles of Human Knowledge, 1734. section 45.
- Fossett, W. (1754) Natural States, R. & J. Dodsley, Pall Mall. London
- The Chautauquan, June 1883, Volume 3, Issue 9, pg. 543
- Scientific American, Apr 5, 1884, pg 218.
- Mann, Charles Riborg and George Ransom Twiss. Physics. Scott, Foresman and Co., 1910, pg. 235.
- "What is Philosophy? - Analysis", Plymouth State University, Philosophy Department
- Bhaskar, R. (2008), A Realist Theory of Science, London: Verso, p.21.
- Bhaskar, R. (2008), A Realist Theory of Science, London: Verso, p.13.
- Marsh, D. (2002), “Marxism”, in Marsh D. Stoker, G. (Eds.), Theory and Methods in Political Science, Basingstoke: Palgrave Macmillan, p.159.
- 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.
- Callinicos, A. (2006), The Resources of Critique, Cambridge: Polity, pp.155-158.
- Watts 1962, pp.111-113
- A Buddhist Bible, "Sutra Spoken by The Sixth Patriarch"