One of the most widely known (and basic) philosophical thought experiments is some variant of “If a tree falls in a forest, and no one is around to hear it, does it make a sound?” Now, I would contend that a far more interesting philosophical question is the one posed by Gary Larson, here, but that is besides the point.
In any case, the “tree falling” exercise, although kindergarten-grade in its simplicity, still at least hints at deeper questions regarding the nature of perception and knowledge, and how these relate to and interact with reality. Can something exist without being perceived? Can we assume that things we do not perceive act the same way they do when we actively perceive them?
Anybody that knows me knows that I approach life scientifically. In an ideal context, there is little difference between science and philosophy (historically, the two were essentially synonymous). Both are simply blanket terms that encompass a variety of outlooks and mindsets that are determined to explain the nature of existence (or some facet thereof) in a logically consistent way. Both science and philosophy break down without this logical coherency.
However, attaining (and retaining) logical consistency is a surprisingly difficult achievement. For both wannabe scientists and philosophers, an actual understanding of logic and how it operates can be elusive. This is what leads to so much idiotic “pop philosophy” (as I call it, lacking a better term); people who think all philosophy amounts to is asking questions like, “If a tree falls in a forest…” and daring you to disprove them (or, for another example, I am sure many of you know or have met someone that took the movie “The Matrix” far too seriously and constantly asks people to “Prove we’re not part of a machine, man” … god, I hate those people).
The shameful thing about this mindset, besides how irritating it can be to deal with, is that it cloaks the interesting and productive philosophical discussion that can be garnered from even simple scenarios like our mysterious tree falling in some distant wilderness.
So, let’s ask ourselves: If a tree does fall in a forest, and no one is around to hear it, will it make a sound?
What must first be clarified as what, precisely, we mean by “sound.” Physically speaking, sound is simple the pattern of variation in pressure propagating through matter (liquid, solid, gas, plasma, or pizza) as waves. A rough analogy is ripples in a pond, through propagating 3-dimensionally. To put it another way, sound is simply movement. When something moves, it displaces the air (or water, or brick if you are the Incredible Hulk) through which it is moving. As that air is displaced, and the movement continues, other air moves into the space left behind, while the air that was originally occupying that part of space displaces more air farther on. This, incidentally, is also the philosophical underpinning of “The Butterfly Effect” (and I don’t mean the Ashton Kutcher movie or the two sequels it apparently has).
This displacement continues based on the magnitude of the original movement. Not surprisingly, a larger degree of movement causes a greater amount of displacement, the waves of which propagate further than is the case with lesser degrees. This displacement can be perceived. In our case, our perception of sound is through our inner ear: (this is a radical oversimplification of how hearing works) the outer folds of the ear channel the displacement to our cochlea, a membrane that oscillates in a pattern consistent with the properties of the sound. The changes in pressure are converted into mechanical movements of the cochlea, which selectively deflect small innervated hairs. These hairs “translate” the movements into electrical impulses, which are interpreted by the brain.
Already, this carries with it implications for the nature of perception. The wave of pressure caused by the displaced air is “translated” through different mediums until it can be processed by our brain as a perceived sound. So is what we call “sound” the actual displacement, or the perception of this displacement as it is processed by our ear and brain? And how do we know that what we hear is really an accurate representation of the sound, considering the processing required for us to perceive it?
The tipping point is whether or not the processing of sound by our ear and brain “faithfully” represents the properties of the original signal. We can see that it does in a number of ways: if you ask completely different people from completely different places, cultures, backgrounds, etc. (to control for extenuating contextual variables) to emulate a certain sound, both will not only do so in a similar way, but most likely be able to identify the object of the mimicry based on the other’s noise. Furthermore, we know that this property can extend across different species with different sizes, shapes, and types of ear mechanism. Parrots, especially, are able to mimic and identify common sounds and voices in a manner consistent with humans, despite the radical differences in the structure of the ear (and brain).
On a more mechanical level, we have devices that can directly measure the properties (frequency, wavelength, amplitude, envelope, etc.) of a sound, and devices that can directly measure the electrical impulses of the ear hairs, and observe the consistency of the correlation between them. That is, the nerve signals modulate predictably with the properties of the sound across different types of cochlea and brains – this research is what has led to the development of cochlear implants that allow deaf people to partially regain their hearing. Similarly, we can artificially create sounds with computers that, with the appropriately calibrated settings, will sound like the “real” sounds, or train computers to recognize particular sounds based on their physical properties. Although their are still limitations with voice activation technology (such as SIRI), this is mostly related to the difficulties of automated LANGUAGE processing, as opposed to a deficit in automated sound processing. This consistency across different types of sensory apparatus indicates that any modification to the signal caused by the mechanical workings of our ear is insignificant.
Now, the more difficult question remains. If there is no receiver to interpret the signal, does it exist? Rather, is it sufficient that we know that the tree falling displaces air, thus causing the physical pressure waves that make up sound, or is there no “sound” until it is perceived? Answering this forces us to address two separate philosophical arguments. First, even though we know that the way we perceive sound is consistent with its physical properties, sound is still defined as a sense, and without anybody around to do the sensing, can we say the sound really existed? A radio signal can be transmitted, but without a radio turned on within its range, nothing happens with that signal.
Let me be evasive for a second and address this question with another thought experiment; a variation of Plato’s cave originally proposed by Frank Jackson in “Epiphenomenal Qualia” (1982):
Mary is a brilliant scientist who is, for whatever reason, forced to investigate the world from a black and white room via a black and white television monitor. She specializes in the neurophysiology of vision and acquires, let us suppose, all the physical information there is to obtain about what goes on when we see ripe tomatoes, or the sky, and use terms like ‘red’, ‘blue’, and so on. She discovers, for example, just which wavelength combinations from the sky stimulate the retina, and exactly how this produces via the central nervous system the contraction of the vocal cords and expulsion of air from the lungs that results in the uttering of the sentence ‘The sky is blue’. […] What will happen when Mary is released from her black and white room or is given a color television monitor?
Poor Mary has all of the physical, theoretical knowledge of color. However, she has never directly experienced color in any fashion. If you argue, as Jackson did, that a signal does not exist without a receiver, than, from Mary’s perspective, colors do not exist until she is removed from the black and white room. Despite all of her knowledge, those colors did not exist within her awareness. This philosophical question can, and in fact has, been answered by scientific inquiry.
What will happen when Mary comes out of her room? For the purposes of the thought experiment, we are forced to assume that she was in there for her entire life, so that she could no have no direct knowledge of color. We know the way brain development works in these situations, and what will happen is that when she comes out of the room she will be completely, or almost completely, color-blind. She will not experience anything new. The parts of the optic cortex responsible for interpreting color information never properly developed due to disuse, so she will remain unable to see color. It is no different than somebody who was born color-blind and never had to spend time in the black and white room. Can we say that color does not truly exist because some people have difficulty perceiving it? That color-blindness can be consistently correlated to very specific types of brain dysfunction is evidence that sensory perception is relatively invariable between individuals. Furthermore, that Mary, due to her theoretical knowledge, will still understand what is meant by “red” and “green,” and be able to determine whether something is of a particular color simply by measuring the light spectra, indicates that there is some objective element involved. If perception were purely subjective, this type of consistency wouldn’t be possible.
Let us for a second, though, ignore the scientific evidence, and take as an assumption that the Mary’s room thought experiment is proof that sensory perception is subjective. That is, “red” and “green”, or sound, or whatever, cannot exist without direct perception. If this is true, then what about those wavelengths of light or sound that humans cannot physically perceive? Ultraviolet, infrared, ultrasound, X-ray, etc. It is impossible for a human to perceive them unaided by equipment; therefore, according to our assumption, they can’t exist. And even once we do measure them, we become vicariously aware of their existence, and are thus perceiving them, only in a different way. Yet if we see a bat flying and catching insects at night, we know it is using echolocation to catch them: it is using sounds we can’t hear. We can observe the effects of that wavelength of sound without being able to perceive it. Or what about colors like brown? It is not the result of a particular wavelength of light in the way that red and green are; brown is just a hue of yellow, orange, and/or red with low saturation or luminance. In this situation, we are perceiving the interaction of yellow, orange, and red light without directly experiencing those colors.
It is simple to verify the existence of things we cannot directly perceive with observation and confirmation with other observers. However, the key here is that the evidence relies in acknowledging the input of other observers, and that the perception of other people, or the bat echolocating above, is as representative of reality as our own perception.
And this is what brings us back to the other philosophical conundrum posed by our question. If we do not directly perceive the sound (whether through our ears or equipment) of the tree falling, how do we have confidence that it happened? Can we assume that events that we do not observe play out the same way as when we observe them? This brings us down the long lonely road to solipsism: as an individual, you can not be absolutely sure about anything except your own existence. So even with the evidence posed by science, solipsism can postulate that only yourself exists, and all of the science is part of your own construction of reality. Therefore, if we are not there to hear or otherwise experience the tree falling, we cannot say that there was a sound, or even a tree or a forest.
Solipsism as an answer, though, is not really satisfactory. First of all, from a logical stand-point, it is the equivalent of dismissing the argument by claiming the argument doesn’t exist. There is no value in doing this; since solipsism is necessarily neither provable nor falsifiable, it doesn’t actually validate or invalidate any philosophical questions, but rather avoids them entirely. You might as well just say that we are all in the Matrix and the color blue is just Agent Smith farting, or whatever (that was the premise of the sequels, right?).
Second of all, solipsism in and of itself is not even a logically coherent argument. It holds that we can not be absolutely sure of anything outside of the self, regardless of how “real” it seems, or how much information we have to confirm its reality. “We” exist as some form of consciousness, and our ability to comprehend the universe is constrained by our consciousness, and thus we cannot truly be sure anything outside of our own consciousness exists. However, this is only a viable point if you hold that anything less than absolute knowledge is worthless; partial knowledge, inference, theory and hypothesis, evidence, etc. are not absolute. We can only absolutely be sure of our own existence, according to the terms of the argument. This forces us to rely on a false dichotomy: that there is only absolute knowledge, or no knowledge at all. The argument underpinning solipsism is inherently fallacious.
Finally, let us for a moment assume as a matter of course that solipsism is not only logically sound, but correct. The sound made by the falling tree cannot be said to exist because it is outside of what we actively perceive. The only thing of which you are absolutely sure is your own existence. However, say you start examining your own brain, the brains of others, and all of the other physical laws and properties of the universe. Since you did not consciously create the universe and its laws, the immense complexity and consistency of reality is an unconscious creation of your mind. Once you acknowledge this, then solipsism as a philosophical argument becomes irrelevant. You are accepting that existence as you perceive it is a system of great complexity over which you have no conscious control, which is identical to a non-solipsist argument that the universe around us is a system of great complexity over which none of us have any conscious control. Essentially, if it walks like a duck and quacks like a duck, there is no reason not to treat it as a duck. A duck the size of the universe.
For all intents and purposes, then, not only did that tree fall in that forest, but it made a sound, even if none of us were around to hear it. Or if we were all just a computer simulation created by oddly nefarious machines using our bodies as an energy source.