What is physicalism?

The importance of in-principle predictability in modern science

The more I read discussions of things like reductionism, emergence, and the nature of the mind, the more I’m bugged by how confused these debates are, both in terms of different writers not connecting with each other and different writers not connecting with the major scientific developments of the 20th century. There’s a vague sense that reductionism is bad and emergence is nice. There’s a vague sense that there are some broad constraints on what kind of views we can have while being consistent with modern science; sophisticated writers in academic philosophy will call these constraints “physicalism.” What any of these ideas really mean, though, is generally unclear. But I think a lot of the confusion could be cleared up if we accepted as a defining feature of the scientific, “physicalist” world view the following principle:

In-principle probabilistic predictability from physics: Given a complete description of an isolated physical system, complete knowledge of the laws of physics, and unlimited calculating power, we could in principle know every way the system could evolve within the laws of nature, and the probability of each possible evolution.

This is intended as a variant on an idea expressed in the early 19th century by Pierre-Simon Laplace as follows:

An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.

The key difference between the the principle I have just formulated and Laplace’s principle is that my principle allows that the laws of physics may, ultimately, be probablistic rather than deterministic. The way most people think about Laplace, this destroys the point of his idea, which was just supposed to be a formulation of determinism. But there is another equally important idea in there, the idea that everything that happens is covered by the laws of physics.

As far as I can tell, IPPPFP is widely accepted both among scientists and among the philosophers who have become our standard sources for physicalism. On the former point, it seems to capture talk of explaining chemistry through physics and biology through chemistry, as well as ruling out some historically important ideas that have ended up in the dustbin of science: IPPFP rules out the idea that there are special chemical laws which only come into play specific arrangements of atomcs, an idea advocated by C. D. Broad of the early 20th-century “British emergentists.” On the latter point, IPPPFP seems to capture what Jerry Fodor meant when he talked about “the generality of Physics vis a vis the special science: roughly, the view that all events which fall under the laws of any special science are physical events and hence fall under the laws of physics.” Or, there is a discussion by Hilary Putnam where Putnam asks if you could explain the behavior of a simple physical system in terms of physics, and says he is willing to allow that “One could compute all possible trajectories of system A… and perhaps one could deduce from just the laws of particle mechanics or quantum electrodynamics that system A could never pass through region 1, but that there is at least one trajectory which enables it to pass through region 2.”

On the other hand, IPPPFP saves us from having to worry too much, at the very start, about notions such as explanation, causation, identity, composition, constitution, realizaion, properties, and events. All of these things cause headaches for philosophers trying to understand the relationship between the domain of the physical and other domains. In the Putnam paper I quoted above, for example, Putnam ends up arguing that the predictability or deduction that he allows for is does not amount to an explanation, that an explanation is something more than that. Similarly, IPPPFP seems to capture talk about the “causal closure of physics” defined as the idea that every event has a physical cause, while allowing us to temporarily set aside worries raised by “causal closure” about whether psychological entities really cause anything.

One important point here is that IPPPFP says nothing about whether facts in domains other than physics are deducible from physics. A neo-Laplacian demon, given a complete physical description of the world, a complete set of (possibly probabilistic) physical laws, and unlimited calculating power, would not necessarily be able to tell you anything at all about, say, the behavior of cats, even if cats are nothing above and beyond the physical. The reason for this is simply that we haven’t granted our demon any knowledge about what physical things count as cats (here, substitute for “count as” your preferred understanding of the “nothing above and beyond” idea–identity, composition, constitution, realization, supervenience, whatever). So it’s a mistake to make the idea of deducing facts in other domains from physical facts central to our understanding of physicalism. You can know certain things aren’t anything above and beyond physics without knowing much of anything else about them. On the other hand, once you sort out the “what counts as what” questions, the relationship between the physical and the chemical, biological, and psychological will be very strong indeed.

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