Purple exists only in our brains
74 comments
·April 4, 2025bryanlarsen
mtlmtlmtlmtl
Agreed. It's simply philosophically sloppy to suggest that purple, brown, and really any other colour don't exist without qualifying what one means by existing in this context, and why that definition of the word is used.
In this case, it seems the "what" is that for a colour to "exist", it has to be a distinct, isolated frequency of light, and (for some reason) it needs to appear in a logical place in the frequency space in relation to other colours. They don't answer the "why" question, and I think if they tried, they would find it difficult, because it's completely arbitrary and in conflict with reality.
Just because some colours do in fact correspond to frequencies of light, that doesn't mean all colours have to. There's a reason we have the terms primary and secondary colour. Some colours are emergent from mixing other colours together. Does that mean they can't appear in the natural world, being discriminated by sensory systems? No. Does purple cause things to happen? I'm sure you could find myriad examples of this in the natural world. Ironically, the only thing here that only exists in our brains, is the notion that purple doesn't exist.
VectorLock
>I'm sure you could find myriad examples of this in the natural world.
Can you think of any?
mtlmtlmtlmtl
Even better, I can tell you one I experienced literally today.
I was at a barbeque, and the people hosting put out one bowl of diced red onion, which despite the name was purple, as it tends to be, and a bowl of crispy fried onion, which was brown. I reached for the fried onion, and avoided the red(purple) onion, because I always hated red onion.
Now, if you want to be annoying, you could make all sorts of red(heh) herring arguments about the cause of me avoiding the red onions being their flavour and aroma in the past having caused me not to like onions, and that's all true. But effects can have multiple causes, and it's also true that the distribution of pigments in the onion leading to an emergent property we dub "purple" did in fact convey information to me that caused me to decide not to eat them. I didn't smell them or taste them. Information was transmitted from onion to mind, via the electromagnetic field. Clearly the emergent property of purple colour has a causal effect here.
As for other examples in the natural world of non-human plants an animals, I don't know, but feel free to use a search engine. But since the colour purple is found in all sorts of life forms(including humans, and all sorts of life forms(including) are able to perceive and discriminate it from other colours, it's fair to assume that the colour purple is out there having all sorts of causal effects.
bryanlarsen
yaky (https://news.ycombinator.com/item?id=43584334) gave a great example of hummingbirds liking purple.
happytoexplain
>All three receptors fire. However, the green receptor has much better sensitivity than the red receptor, so violet triggers the strongest response in blue and red.
I'm confused - it sounds like you're saying it's so off-peak for all of them that, even though it's more proximal to blue, it is effectively equally out-of-range for all three - except that the green receptor is more sensitive. But then that would mean it should look more green, rather than more red and blue, no?
bryanlarsen
More sensitive is a relative term rather than an absolute term -- the green cone selects for its color better than the red cones do.
AlotOfReading
Colors like purple are called "extraspectral" to distinguish them from the spectral colors like violet.
Anon84
Same with pink.
mfeole
I think you're misinterpreting color and wavelength. No color exists "in the wild". Colors only exist in the consciousness of beings that can capture photons. Wavelengths do exist in the wild I guess, or at least it's a human description of how electromagnetic waves move in the wild.
9rx
Purple is a measurement, but wavelength is a measurement too. Ultimately everything is a measurement to us as that is the only way we can experience "the wild".
mfeole
But wavelengths are a property of the waves that exist in the wild, while color is a property of our brains interpreting those wavelengths. There's a fundamental difference there: one could say that wavelengths exist independently of the existence of conscious beings (even if no one measures them), while colors only exist if a conscious being can capture photons and has a certain brain system that can interpret those photon's wavelengths.
bryanlarsen
Color is a mixture of wavelengths of visible light. Mixtures of wavelengths of visible light exist in the wild.
mfeole
No, color is a subjective experience a conscious being has. Different colors each correspond to different mixtures of wavelengths, but that doesn't mean they are the same thing. Color is the thing that you see with your consciousness. The difference is fundamental: wavelengths exist independently of the existence of conscious beings, while colors only exist if a conscious being can capture photons and has a certain brain system that can interpret those photon's wavelengths.
pixl97
Color, as we know them, are a human interpretation of wavelengths and one we know is semi-subjective based on things like color blindness. A creature with less sensitive receptors could see one color were we'd see a much wider range. The reverse is also true that a creature with more sensitive receptors could see far more colors than we do.
erehweb
I think you are misunderstanding the purpose of a headline. "Purple exists only in our brains" is a reasonable summary. Your last paragraph is not a headline.
bryanlarsen
It's a reasonable summary of a portion of the article. I assert that the portion it summarizes is wrong, making the headline also wrong.
bryanlarsen
P.S. Tetrachromats can distinguish between violet and purple, unlike us normal humans.
stvltvs
Not true. Human tetrachromats have an extra kind of receptor somewhere between the blue and red receptors' sensitivity. This doesn't help with colors like violet that are outside of that range.
Also, purple (a non-spectral color) is easily distinguished from violet (a spectral color) if you see them side-by-side.
bryanlarsen
There exists a shade of purple that is indistinguishable from violet because it triggers the cones of the eyes at the same level that violet does.
You can buy paint called "violet". This isn't the spectral violet, it's a shade of purple that looks very similar to spectral violet.
Tetrochromats can distinguish between that purple shade and real violet. But if you mixed the paint using 4 tints rather than 3 you could fool them too.
broof
Violet in the rainbow is not a purple hue, it is a deep blue. Illustrations often color it more purplish but that is inaccurate
stvltvs
Exactly this, purple ≠ violet. They don't even look the same.
You won't see violet on a computer screen because it's a higher frequency than what blue LEDs produce. You won't see it on the output of consumer-grade printers for similar reasons regarding the color of the ink.
The easiest way to see actual true violet is to pass sunlight through a prism onto a white surface.
Purple on the other hand is a mixture of red and blue frequencies that stimulate both kinds of receptors in your eyes. It looks like a reddish blue that can't be produced by any one frequency of light.
True violet looks like a deep, deep blue without any red tint.
bryanlarsen
You usually can't see the really purple-y violet in a rainbow because it's quite dark -- it's easily absorbed by the atmosphere and our color cones aren't very sensitive to that wavelength. But it's there if you amplify it.
automatic6131
More than purple not existing, brown really doesn't exist. Brown is just reflected orange-y light in the right context. You can play around with it, by looking at a picture of say, earthy soil, color picking that pixel and painting it on a white canvas in a different way. Or by trying to shine an RGB light with that color code. It will look orange in a direct light.
Also the purple hallucination is because the same cone that mostly detects red also has a small peak futher along the spectrum next to blue. Since it slightly reacts to blue/violet light, you get that purple tickle.
latexr
Technology connections made a video on it.
automatic6131
Thank you, I couldn't remember who made it, thought it was Knowing Better for some reason
LuciOfStars
Came here looking for this haha, Alec's videos are always a treat.
masfuerte
According to this [1] that extra bump is a myth. But I'm not qualified to judge.
[1]: https://physics.stackexchange.com/questions/673991/red-cones...
hnuser123456
I wonder if near-UV like 380nm starts exciting the red receptor at 760nm like a resonant frequency or harmonic. Would also explain why something that's deeply red enough doesn't start looking purple, because the lower-energy photons can't get the higher energy receptors to trigger, unlike the other way around for deep blue. Which would make the color purple a distinctly quantum phenomenon.
9rx
> brown really doesn't exist. Brown is just reflected orange-y light in the right context.
The Pirahã language only has two words for "color" – which more or less translate to "light" and "dark". Therefore red, green, blue, even orange don't really exist either. Except they do, because we've defined them to exist. Brown is no different.
latexr
Brown is different, because you can only see it in the right context, otherwise it becomes orange. You cannot create brown light sources.
https://www.youtube.com/watch?v=wh4aWZRtTwU&pp=0gcJCdgAo7VqN...
Not naming certain colours does not mean they cease to exist. They can be split, created, and we have the receptors to perceive them.
9rx
> Brown is different, because you can only see it in the right context, otherwise it becomes orange.
But that's how it is defined, so no, it is not different.
moktonar
Well.. no color “exists”, they are just wave like particles.. not even “reality” as we see it exists.. it’s just in our minds.. I think that what they really mean is that there is no wavelength for the purple we see
sandworm101
Except for colors that are combinations of different, disconnected, wavelengths. This is why there are red and blue stars, but no green or magenta stars.
anamexis
That's not really the reason. Green is a spectral color, and in fact the sun peaks around the green part of the spectrum. But to see a star as green, it would need to exclusively emit green light.
temporallobe
This is silly. Everything exists “only in our brains”. The ancient Greeks knew this - see Plato’s Allegory of the Cave.
dboreham
The article says this (and pretty much all the other comments here).
It doesn't however mention James Clerk Maxwell who took a break from inventing equations to torture EE students forever to figure this stuff out, including making the first color photograph.
(Yes I know Heaviside actually came up with the equations we use)
esafak
I understand it more intuitively from a chromaticity diagram, depicting color divorced from brightness: https://en.wikipedia.org/wiki/Purple#/media/File:Line_of_pur...
The curved rim of the horseshoe is the spectral locus; that of the purest, single wavelength spectral colors.
Purples are what you get when you travel between the ends of the horseshoe, where no spectral color lies. This is what is meant when one says purple exists only in our brains; it is non-spectral.
Furthermore, our vision is such that objectively different spectra can look the same; cf. metamers. This follows from the fact that the process of projecting an infinite dimensional spectra into three dimensions (because color is 3D) is lossy. But our vision optimizes the dimension reduction for spectral discrimination in parts of the electromagnetic spectrum that are beneficial for evolution.
perihelions
The spectral locus is highlighted in this one (for clarity),
https://en.wikipedia.org/wiki/CIE_1931_color_space#Chromatic...
That outer *bolded* edge is monochromatic light, as perceived by humans. The number labels are the wavelength in nanometers.
esafak
I forgot to add that in addition to being non-spectral, they are the most saturated colors of their hue. All colors inside the horseshoe are non-spectral; they are mixtures of spectral colors.
yaky
Fun fact: such "purples" exist for other species. There was a study that found that hummingbirds are more attracted to a color that is a mix of green and ultraviolet (imperceptible to humans, but similar to our concept of "purple"), than just green, violet, or ultraviolet.
turnsout
I mean, all colors exist only in our brains. "Color" is a perceptual experience that may start as spectral power and cone excitation, but is then mapped to color sensation through a fantastically complex visual system in the brain. That's why color-based optical illusions work. That's why "The Dress" was controversial.
There's nothing that special about purple. The article makes a big deal about purple "connecting" two opposite ends of the spectrum, but this is just an artifact of humans seeing rainbows and realizing that pink-purple would connect the two ends, even if they're not present in the rainbow. It's just a human framework, not some kind of physical truth.
Put another way, there are a ton of colors that are not in the rainbow, because it's a 1D simplification of our color perception, which is 3D (three cones). Would you say brown "exists only in our brains?"
croddin
This is equivalent to saying a C Major chord only exists in our brains since it isn't a note.
beardyw
Well if we get on to hearing, our ears do a lot better than our eyes. From the source entering into hole on either side of our head we can split the sound into a myriad of frequencies and gather a lot of information from it.
In a similar situation from a single point of light out eyes would say "sort of blue-ish". Most visible frequency information is ignored.
o_nate
It seems reasonable to me to say that soundwaves exist in the world, but music only exists in our brains. There is something added in our perception of the soundwaves that turns them into music.
9rx
Something exits in the world, it seems. Sound waves and music are merely our interpretation of it. Maybe what actually exists in the world is music, and sound waves are what our brain invents when the music is too complex for it to grasp?
o_nate
I guess my point is that sound waves are not terribly controversial. A simple device can measure sound waves present in the air. It would be much more difficult to build a device that told you whether music was playing. Reasonable people could disagree about whether certain sound waves constitute music or noise.
igouy
Color "exists only in our brains".
"It is common to say that certain wave lengths of the electromagnetic spectrum are a given color, but in truth, it is more correct to say that those stimuli are perceived to be of a certain color when viewed under specific conditions. So without the human observer, there is really no color, and practical standards for color must take this factor of human perception into account."
"Charting Color from the Eye of the Beholder" 2005
https://www.americanscientist.org/article/charting-color-fro...
FollowingTheDao
Our brain makes up every color. Color does not exist. Wavelengths exist, but color does not. Our brains turn the wavelengths we see into colors.
Everything is a wave.
bryanlarsen
Does 700 nm wavelength light exist? If so, so does red.
tomxor
Pretty stupid to label something "not real" just because it's not a pure spectral colour.
I guess white isn't "real" either.
larrik
The visible spectrum is the range of wavelengths of light our eyes can detect.
I love the headline, but unfortunately both the headline and the explanation are wrong.
Better explanation:
The red, green and blue receptors in the eyes all respond to all wavelengths of light, they're just more sensitive at red, green and blue respectively.
When violet hits those receptors, it doesn't hit any of the receptors at their peak sensitivity point. All three receptors fire. However, the green receptor has much better sensitivity than the red receptor, so violet triggers the strongest response in blue and red. If the red receptor was more sensitive, we'd probably interpret violet as a shade of blueish-brown rather than of purple.
The headline:
Purple definitely exists. The eye & brain interpret purple and violet the same, but in the physical world they are different colors, and they both exist in the wild. Purple is a mixture of red & blue, violet is the highest visible frequency. A purple flower is a flower that absorbs green and reflects red & blue. A violet flower is a flower that absorbs red, green and blue and reflects violet.