Comment 22 by Bernard Hurley

There seems something odd about this argument. Why couldn't the same thing have been said about the photoelectric effect at the beginning of the 20th century? In this case it did take new physics to account for it.

Yes, but mental functioning is nothing like some unexplained physical phenomena. We know what is involved - particles that obey quantum electrodynamics. We know it's brain cells doing brain cell stuff. There is no explanatory gap.

The argument here cuts both ways. It could be taken as an argument for the absolute irreducibility of mental phenomena as you are arguing that if present day physics will does not account for the mental then no physics ever will. One who thought that such reduction is impossible would agree with you here.

Yes, it does cut both ways. What I'm trying to explain is that it's a bit odd to try to bring in new physics.

If brain cells don't fire because of magic, then if some new physics is needed to account for it then we would expect to be able to find physical evidence for it. In other words it is, in the end, an empirical fact whether there is any new physics.

Yes, but we come up against the success of modern physics and principles such as the conservation of energy. The way we usually search for new physics is to see where conservation of energy seems to be broken. In other words, conventional physics is incomplete. This is how, for example, the neutrino was found. However, you aren't going to find any reputable physicist or biologist who believes that brain tissue somehow breaks conservation of energy. They accept that what brain cells do is pretty mundane in terms of physics. As Sean Carroll puts it, it's all a solution of the Dirac equation.

Therefore, the idea of any new physics comes up against the absence of any sign of it being necessary,

This is a very curious thing to say. Couldn't a similar argument be used to preclude any advances in physics whatsoever?

No, because we are talking about extra ingredients to physics, not new discoveries about how things behave.

'Causal closure' is a somewhat vague term. It seems to mean something like "Every event in the physical universe has a description according to which it falls under some universal scientific law." It is quite possible to accept that and to accept either property dualism or substance dualism.

No it isn't, and causal closure is precise. It means that every physical event has a physical cause. It certainly doesn't work with substance dualism because the extra substance would have to interact with conventional substance, and that breaks closure. It doesn't work with property dualism because property dualism is a load of pig poo. If there are mental properties that co-incide with physical properties, there is no way we could know of them unless they interact with physical properties, and yet the definition of property dualism is that they don't interact. It's equivalent to saying that part of the reason a plane flies is because of invisible undetectable angels. Property dualist aeronautics doesn't work logically, and neither does property dualism.

Another non-sequitur. You might as well argue that because relativity theory is quite good at describing the behaviour of galaxies that are millions of times larger than elementary particles it must be good at describing their behaviour too.

It's not a non-sequitur. This isn't about description, but about new physics, about extra ingredients. We have the ingredients of nature sorted out up to the TeV energy scale. We don't look for new particles at the scale of torch battery energies (which is way above that of the processes in the brain), we slam together beams which have the energies of aircraft carriers.

I still don't see what you are getting at

It's pretty simple. If we express ideas because of physical processes (talking, nerve cell firings), then those ideas must be in our brain cells because something has influenced our brain cells. Talking about ideas isn't an abstract process, it's a physical process. Therefore any new physics must necessarily influence brain cells, and that hits the problem of causal closure, of the Dirac equation.

I will stop there, because this particular argument is at the centre of my book. I can't easily sum up many thousands of words of introduction and explanation in a brief paragraph :)