Saturday, December 29, 2012

Science and metaphysics

by Massimo Pigliucci

Afternoon time at the annual meeting of the American Philosophical Association. I’m following the session on science and metaphysics, chaired by Shamik Dasgupta (Princeton). The featured speakers are Steven French (Leeds-UK), James Ladyman (Bristol-UK), and Jonathan Schaffer (Rutgers-New Brunswick).  I have developed a keen interest in this topic of late, though as an observer and commentator, not a direct participant to the discussion. Let’s see what is going to transpire today. A note of warning: what follows isn't for the (metaphysically) faint of heart, and it does require at least some familiarity with fundamental physics.

We started with French on enabling eliminitavism, or what he called taking a Viking approach to metaphysics. (The reference to Vikings is meant to evoke an attitude of plundering what one needs and leave the rest; less violently, this is a view of metaphysics as helping itself to a varied toolbox.) French wishes to reject the claim made by others (for instance, Ladyman) that a prioristic metaphysics should be discontinued. However, he does agree with critics that metaphysics should take science seriously.

The problem French is concerned with, then, is how to relate the scientific to the ontological understanding of the world. Two examples he cited were realism about wave functions and the kind of ontic structural realism favored by Ladyman and his colleague Ross.

Ontic structural realism comes in at least two varieties: eliminativist (we should eliminate objects entirely from our metaphysics, particles are actually "nodes" in the structure of the world) and non-eliminativist (which retains a "thin" version of objects, via the relations of the underlying structure).

French went on to talk about three tools for the metaphysician: dependence, monism, and an account of truth making.

Dependence. The idea is that, for instance, particles are "dependent" for their existence on the underlying structure of the world. A dependent object is one whose features are derivative on something else. In this sense, eliminitavism looks viable: one could in principle "eliminate" (ontologically) elementary particles by cashing out their features in terms of the features of the underlying structure, effectively doing away with the objects themselves.

The basic idea, to put it as French did, is that "if it is of the essence, or nature or constitution of X that it exists only if Y exists, so that X is dependent on Y in the right sort of way, then X can be eliminated in favor of Y + structure."

As French acknowledged, however (though he didn't seem sufficiently worried about it, in my opinion), the eliminativist still needs to provide an account of how we recover the observable properties of objects above the level of fundamental structure.

Monism. This is the (old) idea that the world is made of one kind of fundamental stuff, a view recently termed "blobjectivism" (everything reduces to a fundamental blob). As French put it, this is saying that yes, electrons, for instance, have charges, but there really are no electrons, there is just the blob (that is, the structure).

A number of concerns have been raised against monism, and French commented on a few. For instance, monism can't capture permutations in state space. To which the monist responds that monistic structure includes permutation invariance. This, however, strikes me as borderline begging the question, since the monist can always use a catch all "it's already in the structure" response to any criticism. But how do we know that the blob really does embody this much explanatory power?

Truthmakers. French endorses something called Cameronian truthmaker theory, according to which < X exists > might be made true by something other than X. Therefore, the explanation goes, < X exists > might be true according to theory T without X being an ontological commitment of T.

Perhaps this will be made clearer by looking at one of the objections to this account of truth making: the critic can reasonably ask how is it possible that there appear to be things like tables, chairs, particles, etc. if these things don't actually exist. French's response is that one just needs to piggyback on the relevant physics, though it isn't at all settled that "the relevant physics" actually says that tables, chairs and particles don't exist in the strong eliminativist sense of the term (as opposed to, say, they exist as spatio-temporal patterns of a certain kind, accessible at the relevant level of analysis).

Next we moved to Ladyman, on "between eliminativism and monism: the radical middle ground." He acknowledged that structural realism is accused by some of indulging in mystery mongering, but Ladyman responded (correctly, I think) that it is physics that threw up stuff —  like fundamental relations and structure — that doesn't fit with classical metaphysical concepts, and the metaphysician now has to make some sense of the new situation.

Ladyman disagrees with French's eliminativism about objects, suggesting that taking structure seriously doesn't require to do away with objects. The idea is that there actually are different versions of structuralism, which depend on how fundamental relations are taken to be. James also disagrees with the following speaker, Schaffer, who is an eliminativist about relations, giving ontological priority to one object and intrinsic properties (monism). Ladyman's (and his colleague Ross') position is summarized as one of being non-eliminativist about metaphysically "thin" individuals, giving ontological priority to relational structures.

One of the crucial questions here is whether there is a fundamental level to reality, and whether consequently there is a unidirectional ontological dependence between levels of reality. Ladyman denies a unidirectional dependence. For instance, particles and their state depend on each other (that is, one cannot exist without the other), the interdependence being symmetrical. The same goes for mathematical objects and their relations, for instance the natural numbers and their relations.

As for the existence of a fundamental level, we have an intuition that there must be one, partly because the reductionist program has been successful in science. However, Ladyman thinks that the latest physics has rendered that expectation problematic. Things got more and more messy in fundamental physics of late, not less so. Consequently, for Ladyman the issue of a fundamental level is an open question, which therefore should not been built into one's metaphysical system — at least not until physicists settle the matter.

Are elementary quantum particles individuals? Well, one needs to be clear on what one means by individual, and also on the relation between the concept of individuality and that of object. This is a question that is related to that old chestnut of metaphysics, the principle of identity of indiscernibles (which establishes a difference between individuals — which are not identical, and therefore discernible — and mere objects). However, Ladyman collapses individuals into objects, which is why he is happy to say that — compatibly with quantum mechanics — quantum particles are indeed objects. The idea is that particles are intrinsically indiscernible, but they are (weakly) discernible in virtue of their spatio-temporal locality. 

Ladyman, incidentally, is aware of course of the quantum principle of non-locality, which makes the idea of precisely individuated particles problematic. But he doesn't think that non-locality licenses a generic holism where there is only one big blob in the world, and that individuality can be recovered by thinking in terms of a locally confined holism. Again, that strikes me as sensible in terms of the physics (as I understand it), and it helps recovering a (thin, as he puts it) sense in which there are objects in the world.

Finally, we got to Schaffer, who argued against ontic structural realism of the type proposed by either French or Ladyman. He wants to defend the more classical view of monism instead. He claimed that that is the actual metaphysical picture that emerges from current interpretations of quantum mechanics and general relativity.

His view is that different mathematical models — both in q.m. and in g.r. — are best thought of as just being different notations related by permutations, corresponding to a metaphysical unity. In a sense, these different mathematical notations "collapse" into a unified picture of the world.

Schaffer's way to cash out his project is by using the (in)famous Ramsey sentences, which are sentences that do away with labels, not being concerned with specific individuals. Now, one can write the Ramsey sentences corresponding to the equations of general relativity, which according to the author yields a picture of the type that has been thought of since at least Aristotle: things come first, relations are derivative (i.e., one cannot have structures or relations without things that are structured or related). If this is right, of course, the ideas that there are only structures (eliminitavism a la French) or that structures are ontologically prior to objects (Ladyman) are incorrect.

So, Schaffer thinks of Ramsey sentences as describing structural properties, which he takes to be the first step toward monism. Second, says Schaffer, what distinguishes abstract structures from the one describing the universe is that something bears those structures. That something is suggested to be the largest thing we can think fits the job, that is the universe as a whole. He calls this picture monistic structural realism: there is a cosmos (the whole), characterized by parts that bear out the structures qualitatively described by the Ramsey translation of standard physical theories like relativity and quantum mechanics. Note that this is monism because — thanks to the Ramsey translation — the parts are interchangeable, related by the mathematical permutations mentioned above.

Okay, does your head spin by now? This is admittedly complicated stuff, which is why I added explanatory links to a number of the concepts deployed by the three speakers. I found the session fascinating as it gave me a feeling for the current status of discussions in metaphysics, particularly of course as far as it concerns the increasingly dominant idea of structural realism, in its various flavors. Notice too that none of the participants engaged in what Ladyman and Ross (in their Every Thing Must Go, about which I have already commented) somewhat derisively labeled "neo-Scholasticism," that is the entire discussion took seriously what comes out of physics, all participants conceptualizing metaphysics as the task of making sense of the broad picture of the world that science keeps uncovering. That seems to me to be the right way of doing metaphysics, and one that may (indeed should!) appeal even to scientists.

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