In a recent interview with Nigel Warburton, neuroscientist Anil Seth mentions (around 3 minutes + 30 sec) John Locke’s distinction between primary and secondary qualities. For primary qualities, the way in which it appears in our experience is pretty directly related to how things are in the world, such as solidity and movement. But there are things like colors which are secondary qualities, where the relationship between what we experience and what’s out there is more indirect and requires the participation of the observer to generate that quality.
But then, at around 6:30 in the video, Anil Seth tells us that all perception works mainly in the top-down, or inside-out direction – from high-level descriptive guesses about the world “down” to details that then fit in to or revise that picture, and from the central nervous system “out” to the periphery. From what little I know of neurology, this inside-out direction of influence is indeed quite important. But that observation threatens, or perhaps we should say trivializes, the primary/secondary distinction. (Seth may well understand this; I’m not sure. The mention of primary/secondary may only be made in order to move beyond it.)
If our perceptions of solidity and of motion are indeed primarily driven from the inside of the brain outward to the periphery, what sense can we make of the idea that our “experience is pretty directly related to how things are in the world”? Our experience is driven from guesses in the central cortical region outward, in both color and solidity experiences. It would seem that all qualities are secondary qualities.
But then, all qualities are also primary qualities, if all it takes to be a primary quality is that it can be specified without reference to an observer. For example, we can define three zones of spectral radiance, one centered at 420 nm, one at 530 nm, and one at 560 nm, each giving less weight to other wavelengths as one gets further from that peak. We can then define “red” things as those whose radiance in that highest-wavelength band bears sufficiently high ratios to the radiance in the other two bands. Of course, I had to lean on human experience of colors to get those wavelength numbers. Yet, I have to lean on human experience of solidity before I could attempt to define that, as well. The alleged primary/secondary distinction is not to be found here.
Seth points out that the solidity of a bus can impact you even when you’re not observing it. OK, but a bacterium which photosynthesizes using only rhodopsin will flourish in green light more easily than in red light of the same total intensity – regardless of whether anyone is looking. Again, no difference here.