We can think of an “empirical theory of the universe” as a triplet of three components:
physical law: the fundamental physical dynamics
initial conditions: the explanatorily fundamental boundary conditions
psychophysics: the fundamental psychophysical laws.
We can shift complexity back and forth between these three while maintaining empirical adequacy.
I’m going to simplify the stories I tell by supposing the psychophysics is epiphenomenalist. If it has two-way interaction, my stories will need to be more complicated.
In my previous post, I noted that you can make physical law extremely simple, without complicating psychophysics much, at the expense of complexity in the initial conditions.
But there are other ways of shifting. You could, for instance, entirely remove dynamics, physical law and initial conditions by adopting a Berkeleian idealism: all the complexity has been shifted into the psychophysics.
You can shift all complexity from initial conditions s0 to dynamics, simply by inserting an extra moment of time before the initial time t0 saying that by law anything at the extra moment transitions to s0 at t0.
It’s more challenging to figure out how to shift complexity from psychophysics to dynamics and/or initial conditions. The best line of approach seems to be to mathematically parametrize all the subjects’ conscious experiences. For instance, to a first approximation, the visual experience of a human could be parametrized by a function from a two-dimensional visual space to a four-dimensional color and depth space, and then the visual experience of n humans is parametrized by a function from two-dimensional visual space to 4n-dimensional space. We could then decompose the psychophysics function ψ(s,t) as ψ(s,t) = Ψ(ϕ(s,t)), where Ψ is a function from the space of mathematical parametrizations of all subjects’ conscious experiences to the actual conscious experiences of subjects, and then a function ϕ(s,t) from the physical world to mathematical parameterizations of experiences. For simplicity let’s suppose that the experiences of subjects are parametrized as vectors in some N-dimensional vector space.
We could then complicate the laws and initial conditions by adding an additional N-dimensional part of the universe containing a single sui generis particle whose location, by physical law, must equal ϕ(s,t) at time t whenever the rest of the universe is in state s at time t. Then we let ϕ*(s*,t) be the position of that sui generis particle when the whole universe is in state s*, and our psychophysical law will be that a universe in state s* at t has psychophysical state Ψ(ϕ*(s*,t)). Since ϕ* is quite simple, that means that we have likely significantly simplified the psychophysical laws. However, there is still the mapping Ψ from mathematical parametrizations to actual psychophysical states. Whatever mysterious complexities this mapping has—here the dragons of qualia lurk—we perhaps can’t simplify.
In any case, thinking about things in this way shows how difficult the data fitting problem is. We have three kinds of moving parts in our empirical theory, and we can shift complexity/tuning between them. Sometimes, a shift will involve a disproportionate rise in overall complexity/tuning, however. Thus, shifting quantum laws to a pseudorandom “digital physics” based on a Turing machine in the Game of Life is likely to result in a system whose overall complexity/tuning is much worse. This means that we have some hope for the project: if we are seeking a theory that optimizes overall complexity/tuning while maintaining empirical adequacy, some options are worse than others.
One way of thinking about the scientific enterprise, then, is that we are simply trying to find the empirically adequate triplet with the least overall complexity/tuning. While not everything is up for grabs, I still fear that at this point we have too many degrees of freedom due to complexity/tuning burden shifts between the elements of the triplet to have much hope of getting at the truth.
I think what we need is something more besides fit to qualia and complexity optimization across the triplet. Here is a suggestion: We need to have a certain limited degree of trust in the “manifest image”. There are multiple ways of working this out, but start with this observation. The idealistic option that folds all the complexity into the psychophysical is something we nearly universally (Berkeley excepted) consider to be a skeptical hypothesis. Similarly, some “digital physics” options look too much like a skeptical simulation hypothesis. It is part of the manifest image that skepticism is false. Thus, we can rule out certain extreme shifts within the triplet. The option that simplifies the psychophysical laws by adding an “experience particle” in a high-dimensional space and pushing the psychophysical law complexity into the physical dynamics also appears to have the flavor of a skeptical hypothesis.
Here is one way of working out the trust in the manifest image. We optimize not just for overall complexity of the triplet, but for overall complexity of the triplet and the distance of the theory from the manifest image, with the distance measured in a metric that weights more heavily the parts of the manifest image that are more deeply ingrained.
Here is another, in a way less flexible, approach. (The two could be combined.) We could take certain especially deeply engrained aspects of the manifest image to be non-negotiable constraints. For example, perhaps, we could take our embeddedness in something very much like three-dimensional space and time as non-negotiable.
Why should we have any trust in the manifest image? Perhaps because it is a part of human rational nature to do so. Perhaps it just is rationally required to trust it, a fundamental constraint on the rationality of dependent rational animals like us.
But there is another question. If we trust the manifest image, we think it has some degree of accuracy. But what explains that accuracy? Here, I think, theism is the main story.