Sounds, colors, heat and cold, according to modern philosophy are not qualities in objects, but perceptions in the mind. David Hume, A Treatise of Human Nature 1738.
The philosophical problem of color crystalizes the mind/body problem. Science says color does not exist in the physical world. But there it is. It’s a case of a clear cut conflict between settled science and our ordinary experience. Reconciling the two is up to philosophy. It’s a hot area right now. Sometimes it seems to put philosophy at odds with science. Simon Blackburn: “the topic is currently wide open, even to the point of some theorists craving a pre-Galilean, Aristotelian, innocent confidence that the world is, in itself, coloured just as we take it to be.”
The start of the color problem is the science of color. Steven Shevell:
Physical wavelengths have no color; instead, we have both detectors in the eye that respond to these wavelengths and, moreover, subsequent neural circuitry that causes our experience of seeing color… Color is in the mind of the viewer (thus psychological), not in light (the physical) or even in the eye’s photoreceptors, which create from light the essential biological signals for seeing… color is a product of the mind… (The Handbook of Color Psychology CUP 2015)
Stephen E. Palmer at UC Berkeley is a leader in the science of human vision, which starts with the physics of light, and continues with physiology and psychology. Here’s how he introduces his discussion of color in his textbook Vision Science:
One of the most fascinating and distinctive aspects of vision is the experience of color. It provides a variety of experiences that are qualitatively unlike those of any other property and are unavailable in any other sensory modality. Physical objects and light sources have the almost miraculous property of appearing to be colored. It is not obvious why this should be; it simply is. People universally believe that objects look colored because they are colored, just as we experience them. The sky looks blue because it is blue, grass looks green because it is green, and blood looks red because it is red.
As surprising as it may seem, these beliefs are fundamentally mistaken. Neither objects nor lights are actually ‘colored’ in anything like the way we experience them. Rather, color is a psychological property of our visual experiences when we look at objects and lights, not a physical property of those objects or lights. The colors we see are based on physical properties of objects and lights that cause us to see them as colored, to be sure, but these physical properties are different in important ways from the colors we perceive…
Newton theorized that sunlight was actually composed of many different “colors” of light rather than just one. He also realized that the “colors” were not in the light itself, but in the effect of the light on the visual system… A physicist can completely describe any uniform patch of visible light in terms of the number of photons it contains (per unit of time) at each wavelength from 400 to 700 nanometers…
Notice that in the physical description of light there is no mention of color at all. This is because, as Newton said, “the Rays to speak properly are not coloured.” Color becomes relevant only when light enters the eyes of an observer who is equipped with the proper sort of visual nervous system to experience it. The situation is reminiscent of the old puzzle about whether a tree that falls in the forest makes a sound if nobody is there to hear it. There may be light of different wavelengths independent of an observer, but there is no color independent of an observer, because color is a psychological phenomenon that arises only within an observer. (Stephen E. Palmer, Vision Science, Photons to Phenomenology MIT 1999, p. 95-97, emphasis in original)
The physical world—the real one outside our minds—contains no color. The description of light by the physicist is complete in that once you’ve got the particles and wavelength, there’s nothing more to say. Color cannot be added to the description. Color “arises only within an observer.”
Palmer does not consider any of this at all controversial. It’s been settled science since the 17th century. The interesting stuff is in the details of the physiology, psychology and cognitive science generally. That gets complicated. This introductory stuff is beyond reasonable dispute. The pre-Galilean, Aristotelian world is gone forever.
While there may be many scientific problems in the details, Palmer sees no philosophical problem. As far as I know, no scientist does. So philosophers who worry about this conclusion are just deluding themselves, right?
Let’s take it slowly. We look at a book on a table. It looks blue. We believe that the book looks blue because it is blue. This belief, however, is “fundamentally mistaken.” All beliefs that objects are colored are fundamentally mistaken. That’s a lot of mistakes people are making. Of course, as a practical matter, everyone makes the same mistakes, so there’s no confusion.
Everyone has visual experiences of colored objects around them. Knowing the physics of light does not change the experience of color. It’s still there. So if the book is not blue, what is?
Color seems to have been driven inside the mind. The only thing that is blue is your “visual experience”. Not the book, not the light reflected by the surface of the book. Color “arises only within an observer,” but where? Your retina is not blue and it is not turned blue by light, regardless of the wavelength. Your retina transduces wavelength information and send it up the optic nerve to your brain in the form of bio-chemical impulses. None of that is blue. The neurons in your visual cortex fire away, but do not turn blue. Nothing in the mind-independent physical world is blue—not outside your body and not inside it either.