If a tree falls in a forest and no one is around to hear it, does it make a sound?

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 riddle that raises questions regarding observation and knowledge of reality.

George Berkeley was a philosopher who created and promoted a theory he called "immaterialism" later referred to as "subjective idealism". His dictum was "Esse est percipi" - "To be is to be perceived". He talked of objects ceasing to exist once there was nobody around to perceive them. In his work, A Treatise Concerning the Principles of Human Knowledge, he 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." One source cites him concisely phrasing the question, "If a tree falls in the forest and no one hears it, did it really fall?" His philosophical musings had nothing to do with sound at all, neither its physical nature nor its metaphysical possibilities.

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 in Volume 3, Issue 9 on page 543 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 on Apr 5, 1884, on page 218 of their magazine 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."

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.

In recent years, however, the quandary has gone back to its possible metaphysical roots, albeit with wording that muddies the philosophical riddle that Berkeley supposed it to be.

The phrase is occasionally the source of parody such as George Carlin's "If a man speaks in the forest and there is no woman there to hear it, is he still wrong?" or Dilbert's "If a tree falls in a forest, and we've already sold it, does it have quality?"

Interpretations

The, perhaps originally intended, scientific answer is that no, the tree does not make a "sound". It makes sound waves, that without an instrument such as an ear, do not convert to actual sound. The common sense answer, as the question has mostly come to rely on, is that yes, it obviously makes a sound since we know that a tree falling makes noise regardless of whether anyone is around or not. The metaphysical, and perhaps first roughly proposed, answer is that no, the tree does not make a sound if no one is there to witness it falling. And indeed the act of making a sound isn't even a quandary, all that matters is that the tree in fact ceases to exist if no one is there to witness it.

The possibility of unperceived existence

Main article: George Berkeley

Can something exist without being perceived? - e.g."is sound sound only if a person hears it?"
The most immediate philosophical topic that the riddle introduces involves the existence of the tree (and its sound) outside of human perception. If no one is around to see, hear, touch or smell the tree, how could its existence occur? What is it to say that it exists when such an existence avoids all knowing? 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. According to bundle theory, an object is merely its sense data. The tree will not make a sound.

Knowledge of the unobserved world

Main article: Observer effect

Can we assume the unobserved world functions the same as the observed world? - e.g., "does observation affect outcome?"
A related question involves 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 a difficult claim to prove. However, many scientists would argue as follows, "A truely 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 dissimilarity between sensation and reality

Main article: Qualia

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 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.

This riddle illustrates John Locke's famous distinction between primary and secondary qualities. This distinction outlines which qualities are actually in an object, and which qualities are not. That is, a red thing is not really red, a sweet thing is not really sweet, a sound does not actually sound like anything, but a round object is actually round (this later an illustration of a tautology).

See also

• Object (philosophy)
• Observer effect (physics)
• Kōan
• Schrödinger's cat
• Epistemology
• Principle of locality

External links

• "What is Philosophy? - Analysis", Plymouth State University, Philosophy Department