Let’s talk epistemology. Actually, let’s use the color purple to bid farewell to epistemology altogether.
By James Vornov, MD PhD
Neurologist, drug developer and philosopher exploring the neuroscience of decision-making and personal identity.
We’ll have to start with the real world as revealed by spectrometers and their ilk. They reveal the electromagnetic spectrum, photons of wavelengths that range from gamma rays (<0.01 nm) down to X-Rays ( 0.01–10 nm) into ultraviolet (10–380 nm) and finally our visible light spectrum (700 to 380 nm), those wavelengths that the photopigments in our eyes absorb and transduce into signals for the visual system. Anything longer, the infrared is heat down to about 1mm. Anything longer than that are microwaves (1 mm to ~1 m) and then radiowaves, which have wavelengths that stretch literally for miles.
Why the narrow 700 to 380 nm, you may wonder. Wouldn’t it be cool to see in microwaves? Get some X-ray vision? They tell me it’s where biology, physics, and our particular environment line up for an optimal photon-based sensory system. First of all, our big blue sun puts out photons across the spectrum, but it peaks in the visible range. So build a visual system based on the most available photons, right? Then the physics of the atmosphere and optics (our biological lensing and focusing) work together to make this visible range most suited for image building. Finally, the chromophores, the vitamin A derivatives that absorb light in our photoreceptors bound to opsins, do their cis-trans shift best in this wavelength. X-rays are too energetic. Microwaves are too weak. The visible spectrum is just right.
We all learned the spectrum in school. The colors of the rainbow: ROY G BIV. Red, orange, yellow, green, blue, indigo, violet. Now it’s seven colors because things come in sevens. Seven seas, seven days. Seven colors. And I’ve thought that they were trying to trick us by naming two colors of shorter wavelength than blue that tend toward purple. We’ll return to indigo and violet in a bit. For now, I want to focus on that classic purple which is a mixture of red and blue. The bottom and top of the spectrum.
Like this sunset where blue and red are scattered by the atmosphere and combine to make all kinds of reds, magentas, purples, and deep blues. If you pointed a spectrophotometer at the sky, it would tell you the purple areas are varying mixtures of blue light and red light wavelengths.. There’s no purple in nature; it is a mix of red and blue.
Where purple is imagined
So let’s turn to how color is detected in the retina by photoreceptors. We have three types of cones which we call S, M, L for Short, Medium, and Long. The peak absorption of these is at S-cones peak around 420 nm (deep blue/violet).
M-cones peak around 530 nm (green). L-cones peak around 560 nm (yellow-green, not red!).
In V4 this mess is the colors we know well: blue, green, orange, yellow, red. And purple. The brain solved this a long time ago.
We’ve done it ourselves with digital cameras: Bayer pattern. Below is the graph of sensitivty for the Red, Green, and Blue sensors of a typical digital camera sensor. It really talkes all of sensors to cover the spectrum, but green sensitivity in cameras is best. Because of the overlap and varying sensitivities you can’t just take the sensor output and see an image that matches the environment. You need a lookup table, a LUT, for every sensor that provides a calibrated translation from levels of RGB into every color in the color space. There’s some art in that translation from pixel value to color sense and different camera manufacturers tend to have their look. But that’s why you can’t look at a RAW file without conversion.
Really not purple, but magenta. And cyan too.
So that’s how you see purple. As I said in the sub, I want to simply get there and say I’ve solved epistemology. There’s what’s out there that a spectrograph records. Then there’s what V4 codes, which is what we experience in here. Easy peasy.
Now, one clarification. I keep saying purple, but I’m sure you’ve heard of magenta. The color we see as blue plus red minus green is actually magenta. I’ve seen magenta as an ink color. So really, this non-spectral purple is really magenta. When those V4 neurons respond to blue plus red, no green, it’s really that pinkish magenta color it’s responding to. We use magenta ink in printing because it’s a subtractive color. It reflects blue and red, absorbing green. So it helps in providing a full range of colors that are in the environment, not just those in the spectrum. When we have additive color, like an RGB monitor, where we can turn on red and blue and leave green out.
So our engineers have figured out how to express colors, including red plus blue without green. And on the diagram, you’ll note another non-spectral color, cyan. Cyan reflects blue and green but absorbs red. So just like magenta is anti-green, cyan is anti-yellow, being blue plus green minus yellow. And so we can print more than just the spectral colors with these invented colors of magenta and cyan. And in V4, the brain also has this same way of representing the non-spectral color of red plus blue, no green as magenta. And yes, as part of the opponent process coding in V4, we have cyan preferring neurons. In the end, we have the following known CIE diagram which shows, as well as your phone or monitor can display, all of the colors that are available to human vision.
I should point out that as primates, we’re very privileged mammals having rich trichrome color vision, in particular, old world primates including humans. Reptiles and birds? Actually, tetrochromatic or better. Pigeons, certain butterflies, and the mantis shrimp have 5 or more distinct photoreceptors. The mantis shrimp, famously with 12–16 photoreceptor types, shows surprisingly poor color discrimination in behavioral tests. And for range? We see to around 400 nm, mostly because the lens and cornea block UV. When I had cataract surgery and intraocular lens replacement, I realized that I was seeing sharper and richer blues than before. Birds and many insects see all the way to 300 nm, well into ultraviolet. And on the long side, we see to 780 nm or so, but as you probably know, it’s perceived as heat after that, so we can’t see but feel. When photons get down to that energy level, they’re not so useful for imaging, but we can feel it, and some animals like snakes have specialized heat sensors, obviously useful for prey detection.
So we have a richer visual experience than our dogs, but birds and insects have a very different set of inputs. Which just goes to show that our world is limited by our senses.
Kant was right about a priori knowledge
And so we’ve put epistemology to bed. There’s the world outside the skull, which we perceive through our sensory receptors. Our brain then uses the information, transforms it, and uses it to navigate and ultimately control interactions with the environment. We didn’t imagine or invent magenta; the primate brain, as a trichromat, needed to represent blue plus red in the absence of green as a feature in the environment.
Magenta, our perception of color, is a priori knowledge about the world. We’re born with it. And so, in the history of philosophy regarding epistemology, we see that Kant was absolutely correct. A nod to Plato as well for his recognition that some perceptions are not in nature, but outside of it. His “Forms” were imagined as abstract and eternal. He couldn’t say where that was, but at least not in the world,.
However, what Plato, Kant and philosophy up until now was missing is just access to modern neuroscience. Kant was right, there is e a priori knowledge, but it’s simply the connections programmed during brain development. It’s fine-tuned and adjusted after birth, once visual information from the real world becomes available. We’re born knowing about magenta and cyan. We’re born with the circuitry of V4 that decodes the mess of color sensitivity from the photopigments of the S, M, and L cone types. We have no need to figure out color. If we did, everyone might have a different way of putting the evidence together and some might never get it. Fortunately, we’re born with color perception as a build in feature. This is a priori knowledge, built into the brain.
And if we built purple from nothing, what else are we born knowing? Good and evil? Fairness? The existence of God? Next time maybe.
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© 2025 James Vornov MD, PhD. This content is freely shareable with attribution. Please link to this page if quoting.
