Reduction, Wavefunction, and Physics

Ok, so this is about reduction and Wavefunction, but it isn’t about the quantum measurement problem. Go figure.

Following up on my comments here, I generated another overly long reply. And for some reason I can’t get the reply to post over at The Curious Wavefunction, even when I try to break it into smaller bits.

So I’m putting it here:

@ Wavefunction

OK, we agree that one form of reduction fails: It is not the case that all biological facts can be reduced to physical laws. Indeed, almost nothing can be reduced to the physical laws alone.

Do we also agree that biological facts are reducible to a much broader category of physical facts? That is, if in addition to the laws we also include all of the actual physical properties (the complete physical state at all times), then all the facts about biology will be fixed. So once you know everything about the particles (so to speak), then you can – in principle at least – deduce everything there is to know about giraffes, hearts, and so on.

It seems that at least some of our disagreement is over what it means to “reduce” something to physics. I don’t know whether it’s helpful to point this out, but scientists and philosophers typically mean different things by “reductionist.” Scientists typically call something a “reductionist account” if the relevant mechanism lies at some lower level (molecular biology, say). If instead the relevant mechanism involves whole populations or ecosystems, then the account is said to be “anti-reductionist” or “holistic.”

Philosophers, on the other hand, are more interested in whether the higher-level facts can be derived from the lower-level facts (or whether there are higher-level causes that are independent of physical causes). Most scientists think that the philosopher’s reduction is an obvious truth and so isn’t even worth considering (it just rules out magic and ghosts, they’re inclined to say), and so they instead focus on questions of complexity, emergent regularities, etc. Which is all well and good, as long as one isn’t thereby seduced into thinking that structure, function, biology, etc. somehow go beyond the purely physical (which is the philosopher’s notion of reduction).

There’s also a lot that could be said about different philosophical varieties of reduction. Nagel, for example, was mostly interested in the reduction of one theory to another. This would tie in with your attention to laws (and seeming indifference to states/properties). But the examples you embrace (e.g., the existence of giraffes) don’t seem to fit in too well here, since they seem like specific happenings rather than general laws.

In any case, I’m a hard-core reductionistic physicalist, but even I don’t buy into the claim that you can derive one theory merely from the laws of another theory (and some “bridge principles”). It seems obvious that you’ll also need to know some details of what actually happened – that is, you’ll need to know some physical details in addition to the laws.

A superintelligent freak could list all of the countless events that would encompass the random mutations and effects of chance that we are talking about. But it would be impossible to assign a priori probabilities to all these events and predict that the net probability of our current universe existing is 1. This would be possible only if the superintelligent freak knows the entire future of the cosmos, in which case the discussion becomes meaningless and unscientific.

If physics is deterministic, then knowing the complete physical state and the physical laws does indeed provide knowledge of the complete future of the cosmos (setting aside epistemological issues of computability, possibility of measurements, etc.). I don’t see how this in any way makes the discussion “meaningless” or “unscientific.”

Now, if you’re not interested in ontology, but instead in what we very finite and flawed creatures could come to know, then of course our knowledge of physics is going to tell us little about biology. I think we all agree on this. But this is due to our ignorance of some relevant physical facts (e.g., how some DNA will – as a natter if fact – recombine during a particular case of meiosis); it is not due to some biological fact that goes beyond the physical facts.

Of course, if physics is indeterministic, then knowing the complete state at a time and all the laws still won’t allow one to precisely predict the future. But even in this case, there’s an important physicalist principle that holds: One will never be able to make a more accurate prediction using a higher-level science (e.g., biology) than one would in principle be able to make using the complete (indeterministic) physical account. Any higher level account is going to leave out some physical details, and therefore will be less accurate (though perhaps more illuminating) than the physical account.

This is why I find it very misleading to claim that a reduction to physics fails because the existence of some things depends on chance events. To the extent that the claim is correct, it is completely irrelevant to understanding the relationship between physics and higher-level sciences like biology.

The key point is that scientific laws are generally not in the business of telling us what exists. Laws (for the most part) tell us “If you start in state A, then you will end up in state B.” This is true of physics, but it’s also true of all other scientific laws. Laws don’t tell you what exists; they just tell you how one state time-evolves into another.

So if physics has a problem with chance events and historical contingency, than so does every other scientific law – but really it’s not even about “chance” or “contingency” at all, it’s just a simple fact that the laws of nature are (as best we can tell) compatible with many different histories of the universe. There’s more to the world than just the laws. (If the world is deterministic, then there’s also the initial conditions. If the world is indeterministic, then we also have the outcomes of all stochastic processes.)

So if by “physics” we mean to include physical descriptions of actual physical stuff around us (let’s just pick some hydrogen atom in space), then physics will indeed include an account of “historical contingency.” Even in a deterministic world, that hydrogen atom is there because of the history of the universe. In an indeterministic world, there will also be chance physical events. So a complete physical description will also take “accident” into account in that it will include the outcomes of stochastic processes.

But once we recognize this, then there seems to be no reason to single out particular biological features (such as the function of the heart or the existence of giraffes) as being “not reducible to physics” just because they appeal to some historical fact in addition to the laws of physics. This doesn’t distinguish functions, or biological features, or complexity from anything else. All we’re saying is that the laws don’t tell us what the actual physical state is.

Thus pointing out that the “environment itself is a result of countless chance events and encounters,” seems to be beside the point. If the super intelligent freak (or Laplacian demon) knows all the physical facts (including the outcome of any indeterministic physical processes), then the freaky demon will know everything there is to know about the environment, and the organism, and the population the organism belongs, to and the complete history of that population, and so on.

This is why I’m suspicious of your claim that “a superintelligent freak could list all of the countless events that would encompass the random mutations and effects of chance that we are talking about. But it would be impossible to assign a priori probabilities to all these events and predict that the net probability of our current universe existing is 1.

Of course the freak would be unable to assign any useful probabilities without knowing the actual state of the universe. But why should we suppose that physics is about “a priori probabilities” (to use your phrase)? Physics is about transitions from one state to another state. Neither or knowledge of the laws nor our knowledge of the states is a priori. And, of course, no one would expect that you could derive even the probabilities of a final state without knowing something about the initial state.

Further, I see no reason to suppose that a reduction of biological facts to physical facts would require determinism (i.e., the claim that given the actual initial state, the probability of landing in the actual current state is 1). Let’s allow for an indeterministic quantum physics. Then the complete physical story is going to include the outcoc

The key point as far as physicalism goes is that given the outcome of indeterministic physical processes, there’s no further leeway regarding biological facts. It’s never going to be the case that a freaky demon knowing all the biological facts will be able to predict some future event more accurately than will the freaky demon who knows all the physical facts (both the laws and the complete physical state). Thus physics is the most complete, most accurate, account of our world, and there’s no extra wiggle room left once the physical facts are given.

I haven’t read Kauffman, but usually the complexity folks are particularly interested in how the behavior of some systems is largely (though obviously not completely) independent of initial conditions, whereas the behavior of other systems depends sensitively on the precise details of the initial state (e.g., chaotic systems). There are many interesting issues to pursue here, but think we only obscure them if we suggest that they somehow take us beyond physics.


2 responses to “Reduction, Wavefunction, and Physics

  1. Short version (that I left chez Wavefunction): No scientific law deals with actual historical events; laws are about regularities. So your complaint has nothing in particular to do with physics.

    Once we include actual historical events (i.e., the physical state) with the laws, then physics has no trouble accounting for everything in the actual world.

  2. Pingback: Does Reductionism Explain Anything? « Physicalism

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