In this discussion of whether color experience can be reduced to the physics we have, let us return to the beginning and at least try to agree on why we disagree.

Before I knew about science, I lived in a world which included colored objects. Subjectively I still do. But I have also learned a theory of the world according to which it is made entirely of particles and fields. For the purposes of discussion, we can say that according to this theory, the world consists of particles and fields in a changing spatial configuration. (If quantum mechanics comes up, let us say that it consists of an association of complex numbers with a set of such configurations.)

I expect that we agree that at the fundamental level, this theory does not contain color. So if I am to believe that this theory describes the whole of reality, and if I am to regard my experience of colored objects as part of reality, then the elements of that experience must be identified with complex, non-fundamental entities or properties appearing in the physical theory.

A priori, I regard it as outlandish that color is an arrangement of things in space, or any other such composite property from the physical theory in question. They appear to be radically dissimilar things, as if one were to say that yesterday was the number 2. So when someone says that color is such a property, if they wish to convince me, they must overcome this skepticism and somehow explain how this can be so. I mention this, not to signal that I shall not be moved come what may, but to mention a psychological fact, and to indicate how large the gulf between the one and the other appears to me.

Robin Z, Richard Hollerith, and Silas Barta have all defended the physical reduction of color, as have several others. So I shall examine their positions in turn, and at least try to understand what it is that they are proposing.

Robin Z's most recent statement of her position is here. She says the experience of greenness occurs in the mind, the mind is a product of the operation of the brain, and the ultimate cause of the experience of greenness is usually the incidence of light of a certain wavelength upon the surface of the eye, which produces an unspecified causal sequence resulting ultimately in the experience of greenness.

This explanation does not specify which complex physical entity is the experience of greenness, or how it is that color arises from colorless physics. So it does not help me understand or give me reason to change my view.

An analogy with the figures in a video game is also presented. But described physically, an image on a screen is an arrangement of things in space, so it is not a problem to account for it in terms of a spatially based physics.

In Richard's comments, I cannot find a specific account of what color is physically. There is the assertion that "The state of my brain that corresponds with the blue experience can be a normal, ordinary, conventional physical state", but there is no explanation of what sort of physical thing a "blue experience" might be.

Richard also talks about information processing and model maintaining, so that is probably relevant to his concept of how the reduction is to be achieved, but it is not enough detail for me to work with.

Silas has been more specific. Following Gary Drescher, he refers to the "generated symbols" of the Lisp programming language. Silas says: "To experience blue is to feel your cognitive architecture assigning a label to sensory data."

We can all agree that there is no barrier in principle to providing a physical account of what happens inside a computer when such an operation occurs, though the details may be complicated. For example, in the computers we have, such a process consists of many charged objects changing their locations inside a number of transistors in a certain way.

I have no objection in principle to Silas supposing that an analogous computational process might occur in the human brain. The physical reality there will similarly consist of numerous charged objects moving around, such as ions moving across membranes, in a certain way.

Silas says that the experience of color is how it feels for this to happen. But I still do not see where the color is. Either I am to look for it in the motions of the ions themselves, in which case I do not see it; or I am to look for it in the "feel" of those motions, but I do not know what that means, in terms of the physical theory with which we began.

I therefore conclude that I have not yet been given a reason to think that color can, after all, be found in a colorless physics.