Nearly everybody nowadays accepts the 'causal completeness of physics' - every physical event (or at least its probability) has a full physical cause. This leaves no room for non-physical things to make a causal difference to physical effects. But it would be absurd to deny that thoughts and feelings (and population movements and economic depressions . . .) cause physical effects. So they must be physical things.

Nearly everybody nowadays accepts the 'causal completeness of physics' - every physical event (or at least its probability) has a full physical cause. This leaves no room for non-physical things to make a causal difference to physical effects. But it would be absurd to deny that thoughts and feelings (and population movements and economic depressions . . .) cause physical effects. So they must be physical things.

A century ago mainstream science was still quite happy to countenance vital and mental powers which had a 'downwards' causal influence on the physical realm in a straightforwardly interactionist way. It was only in the middle of the last century that science finally concluded that there are no such non-physical forces. At which point a whole pile of smart philosophers (Feigl, Smart, Putnam, Davidson, Lewis) quickly pointed out that mental, biological and social phenomena must themselves be physical, in order to produce the physical effects that they do.

The orthodox view of colour experience assumes that, when we see a colour difference between two surfaces viewed side-by-side, this is because we have different responses to each of the two surfaces viewed singly. Since we can detect colour differences between something like ten million different surfaces, this implies that we are capable of ten million colour responses to surfaces viewed singly.

There is a brain mechanism that works to identify colour differences directly, without first identifying the absolute colour of each surface. So on my view there is no reason to suppose anything like ten million colour responses to surface viewed singly.

I think that there are non-physical laws all right: genuine (if not strict) laws written in the language of biology, economics, and so on. But I don't regard that as a contentious issue. Even reductionists about chemistry will think that there are special chemical laws whose formulation makes essential use of chemical terminology.

If neuroscientific research shows that those mechanisms only contain comparative information about colour differences, and have 'thrown away' more fine-grained information about the absolute colours of single surfaces, then that would support my position, in a way that just introspecting our colour experiences can't.