On the causal completeness of physics

by Massimo Pigliucci

As readers of this blog know, I am not sympathetic to extreme reductionism, and reject both it and determinism in favor of a robust concept of emergence. Of course, I think a moderate epistemic reductionism has been the winning approach for science, and I agree that there is plenty of room for some degree of ontological reductionism (i.e., some times the whole is just the sum of its parts). But I’m not a fan of the everything-is-an-illusion-because-of-reductionism school of thought that seems so popular among skeptics and some scientists these days (see, for instance, my take on Alex Rosenberg’s particularly sharp articulation of that position).

Now, during my participation in the recent workshop on philosophical naturalism organized by Sean Carroll, I had an interesting exchange with physicist Steven Weinberg, who played what he thought was a trump card in favor of reductionism “all the way down”: he mentioned the causal completeness of the laws of physics. I asked him to elaborate on the point, and he said that the laws of Newtonian mechanics, for instance, are causally complete in the sense that there is no room within the equations for any unaccounted parameters. It follows, according to Weinberg, that those equations are a complete description of the causality of the system, leaving no room for emergent properties.

Okay, so the prima facie objection here is that Newtonian mechanics is known to be wrong, so it’s a bad example to make a case for extreme reductionism. Second, mathematically, Newtonian mechanics can be derived as an approximation of relativity theory, which means that — as it turns out — there were some parameters missing from Newton’s equations after all. Third, one can immediately raise serious philosophical issues concerned with the very meaning of “causality” being deployed here, and which specific form of theoretical reductionism Weinberg thought he was defending. But I figured that the guy knew what he was talking about (after all, I’m not the one holding a Nobel in physics!), so I took note and postponed further thinking on the issue until I had time to look into the primary literature on causal completeness. Now I have, and it turns out that things are more complicated, and interesting!

In this post I’ll comment in some depth on a paper by Agustín Vicente published in International Studies  in the Philosophy of Science (2006).

To set the frame for the discussion I cannot do better than to quote Vicente’s abstract verbatim: “According to an increasing number of authors, the best, if not the only, argument in favor of physicalism is the so-called ‘overdetermination argument’. This argument, if sound, establishes that all the entities that enter into causal interactions with the physical world are physical. One key premise in the overdetermination argument is the principle of the causal closure of the physical world, said to be supported by contemporary physics.”

In turn, the overdetermination argument goes like this (again, from Vicente), where “dubious” events are events that are alleged to be non-physical (such as mental causation, supernatural interventions, and the like):

(i) The principle of the causal closure of the physical (CCP): every physical effect (i.e., caused event) has physical sufficient causes;

(ii)  Causal efficacy of the “dubious”: dubious events cause changes in the physical world;

(iii)  No overdetermination: there is no dubious/physical causal overdetermination.

And the conclusion is that:

(iv) Dubious events are physical events.

As stated, I have no trouble with either the CCP or the overdetermination argument. I am a physicalist* [1], after all. But notice that the CCP as stated by Vicente bears only a family resemblance with what Weinberg invoked in our discussion. The CCP doesn’t state anything like that the laws of physics as we understand them now are causally complete. It simply states that, again, every physical effect has a sufficient physical cause. As such, the CCP excludes mental dualism (in philosophy of mind), vitalism (in biology) and supernaturalism (everywhere), but does not exclude emergent properties, as long as these are conceived as qualitatively new physical properties that manifest themselves under certain conditions of complexity and organization of matter. Another way to put this is that the CCP leads to physicalism, but physicalism does not logically entail extreme reductionism.

But why should we construe an argument like the one above in defense of physicalism to begin with? Vicente gives an interesting answer right at the onset of his paper. He says that it used to be thought that physicalism was a reasonable inductive inference arising from an historical trend of unification among all the sciences. That is, an epistemological argument could be made that, just like chemistry eventually reduced to physics (something about which not everyone agrees, actually), so eventually biology would be reduced to chemistry, and the social sciences would fold into biology. Et voilà le jeux sont faits, so to speak. Not so fast, as it turns out. There is an increasing agreement among philosophers (and, indeed, even Weinberg assented when I mentioned it!) that the actual history of science is taking a different trajectory, with the special sciences becoming more, not less, independent of physics, each operating at its own level(s) of complexity and explanation, with no hope of a true “theory of everything” coming along, ever.

That being the case, then, supporters of physicalism have to come up with an ontological account instead (which, again, is precisely what Weinberg tried at the naturalism workshop). This would be a theory that argues that while epistemic reductionism is practically impossible, ontologically speaking, it’s quarks (or strings, or whatever) all the way down. Which further implies that whatever laws describe the behavior of quarks (or strings, or whatever), they are the only laws of the land (and by land, I mean the universe), everything else is ontologically superfluous. Hence, the overdetermination argument above, which uses the CCP.

I could quit here and declare victory over Weinberg: the CCP that he invoked does not do the work that he thinks it does (specifically, eliminating the possibility of emergent properties), although it does do the job we both want it to do (eliminate dualism, vitalism and supernaturalism). But no, I just had to go on and read the rest of Vicente’s paper, didn’t I?

The question he goes on to address is an obvious one: why should we believe in the causal closure principle, particularly since it most certainly isn’t a law of physics? There are two classes of reasons: we could think of the CCP as a methodological principle guiding physicists (and, really, naturalists of all sorts, particularly skeptics) in their actual practice. Think of it as the Scooby-Doo principle: whenever you think there is a “dubious” (i.e., paranormal, extranormal, supernatural) effect at play, by the end of the episode it will turn out that it was just plain old physics (or biology, or whatever other science ends up offering the solution to the mystery). Alternatively, we could say that while the CCP is not strictly a law of physics, it is somehow supported by the laws of physics, and that’s a hell of a support!

By the end of the paper Vicente concludes that both lines of inquiry are fruitful. On the one hand, there are good inductive reasons to think that causal completeness is a valuable methodological precept. On the other hand, the principle can be connected to the laws of physics, and specifically to the laws of conservation. The arguments Vicente makes in the bulk of his paper are complex, and he does an admirable job at pointing out the difficulties of each. I will simply mention some of the highlights, to give you a flavor of what I think is a truly interesting and well written philosophical paper (one that Weinberg would do well to read, before talking about causal closure again, in my modest opinion. Ok, ok, that opinion wasn’t really that modest...).

One of the interesting notions emerging from Vicente’s historical analysis of the CCP is that it has not always been held during the history of science. Something like the CCP was entailed by atomistic-Epicurean physics, and then much later on by Leibnizian dynamics, but not at all times in between. Still, contemporary physics certainly does accept the closure principle as a strong methodological precept.

Vicente gets himself into what I think is unnecessary trouble, however, when he discusses emergentism. Here is what he says, verbatim (p. 153 of the article): “Emergentists would agree that physics is the science of the bottom level; nonetheless, they would claim that it cannot explain everything that happens in its domain, for some causal powers ‘emerge’ and bring about changes in the physical world that physics cannot explain. ... Some authors, such as Cartwright and Dupré, deny that physics is basic in the sense used here, that is, that it explains and describes the bottom level that somehow fixes or determines the rest of the facts of the world.”

But none of that seems to me in contradiction with the closure principle, as long as we understand emergent properties as physical properties, described by physical (or biological, or even social) laws or law-like generalizations. Remember, the CCP simply says that every physical effect has physical sufficient causes. To go further and somehow construe emergent properties as problematic for the CCP (or, as Weinberg would have it, the other way around) is a non sequitur. For instance, the best studied emergent properties are those pertinent to phase transitions (from solid to liquid, liquid to gas, etc.). But even if it turned out that the theory of phase transitions is irreducible to lower-level physical theories (because of the appearance of truly qualitatively new physical phenomena) we would still be talking about physical processes. Nobody is invoking a type of solid-liquid dualism, and certainly no one has suggested that the transition between liquid water and water vapor is the result of supernatural intervention!

Back to the main track. Vicente at one point (p. 154, if you are reading along) concludes that there is no good a priori justification of the closure principle, for three reasons: “(i) it cannot be justified by assuming reductivism, (ii) it does not follow from the fact that physics is a basic science (emergentists assume this, but they deny that it is explanatorily comprehensive), and (iii) it is possible to argue that physics is not basic.” He then goes on to explore the possibility that the CCP might be justifiable a posteriori, by induction.

Here again, the verdict is mixed. On the one hand — and despite the above mentioned discontinuous appearance of the closure principle in science throughout its history — it simply cannot be denied that the CCP has worked very nicely for physics: whenever physicists have looked for a physical explanation of a phenomenon, they found it. (Or, as Tim Minchin observed in a different context: “Throughout history every mystery ever solved has turned out to be, Not Magic.”) But theoretical biologists like Stuart Kauffman have also long pointed out that physico-chemistry has not at all been quite as successful at explaining biological phenomena. Kauffman and others (myself included) impute this failure to — you guessed it! — emergent properties arising from the interactions of complex systems of molecules, cells and even whole organisms. But once again I just don’t see why Vicente seems to think this is somehow a problem for the CCP understood as he presents it at the beginning of the paper. Kauffman and colleagues are most certainly not arguing for vitalistic forces, and much less for supernatural ones. But perhaps I am missing something fundamental here.

Finally, we get to the possible connection between the closure principle and physical laws as we understand them. According to Vicente there are two prominent venues of inquiry here: causal closure may be related to the action of forces in physics, or it may be connected to the idea of quantity conservation (such as the conservation of energy, or of momentum). Let’s take a quick look at both.

First off, notice that the discussion at this point is about what constitutes a cause (we are talking about the causal closure principle, after all!), a notoriously treacherous territory in philosophy, as much as it is often blissfully ignored by scientists.

Be that as it may, one basic idea (developed originally by David Papineau) is that physics has been able to expand its explanatory domain by invoking a smaller and smaller number of forces (currently, three: electroweak, strong, and gravitational). Ergo, there is no reason to think that in the future we are going to need more forces to augment our explanatory power (indeed, there are reasons to believe we’ll need fewer: that’s what the so-called “theory of everything” which physicists have been after for a while is supposed to do, to unify all the remaining forces in physics by way of a unitary account).

To make a somewhat long (but fascinating) story short, here is how Vicente summarizes the situation for the CCP in terms of its connection to forces in physics:

(i) Physical effects are, or involve, variations in the quantity of (the universally conserved) energy possessed by an object (body or whatever); 
(ii) The causation of physical effects consists in the action of forces; 
(iii) There is inductive evidence, partly negative, for the view that such forces are physical forces. 

Considering (i), Vicente points out that, in principle, the CCP could be questioned on the ground that not all physical effects involve a variation in the quantity of energy. Admittedly, though, it is hard to imagine which physical effects would fall within this unusual category.

Skipping for a moment to (iii), it can be questioned, in principle, by dualism, vitalism and emergentism, on the ground that its conclusion is inductive (and therefore tentative). As I said repeatedly above, however, I can’t figure out why Vicente thinks that emergentism is in the same category as the first two. All emergentism says is that some forces (or phenomena, more broadly) are not fundamental, that they only manifest themselves at certain levels of organized complexity. But these forces or phenomena would still be physical.

We are then left with (ii), the logical link to forces. Vicente doesn’t seem to like the invocation of forces to buttress the CCP, on at least two grounds: first, talk of forces actually smells a bit too much of classical mechanics, which has been replaced by a more sophisticated physics, to the point that physicists themselves may one day abandon any talk of force whatsoever, re-conceiving the whole shebang in terms, say, of fields and associate particles (Higgs!). He also claims that forces actually already play a secondary role in physics, with the main stage being occupied by conserved properties anyway.

Which is exactly where we turn at the end of this tour de force. The basic idea here is that causation is just the transfer of a conserved quantity; indeed, forces — in this conception — are not causes at all. Note that not all physical quantities can be transmitted (velocity, for instance, cannot). But if they can’t, then they don’t have causal powers (if you are thinking that a fast car hitting you does have causal power, you have to remember that the force of the impact is due to a transfer of another quantity, kinetic energy, not to the transfer of velocity).

While there is a certain intellectual and aesthetic appeal to defining causes simply as transferences of conserved quantities, so-called “CQ” theories of causality are just as controversial as theories of causality based on forces (I told you, causality is a mess!). I will leave it to the reader to work through the last part of Vicente’s paper to appreciate the nature of the controversy, but before closing I need to note that Vicente does admit that it is not possible to deductively exclude the existence of unknown forces, or of unknown quantities that can be transferred and are overall conserved. This is really not a particularly strong objection to either force/CQ accounts of causality or to the CCP itself. But it does mean that we arrive at the rejection of, say, new forces responsible for astrological effects (one of Carl Sagan’s favorite arguments against astrology) only inductively, and therefore in a potentially fallible manner. Not that any physicist or physicalist ought to be losing sleep over such matters, of course.

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[1] The * after physicalism in my case is because I actually believe in an entirely natural ontology, but not necessarily an entirely physical one. For instance, I am attracted to mathematical Platonism. But of course mathematical objects (say, numbers) don’t violate the CCP because they do not have, per se, any physical effects.