The coherence of physicalism: are there any solutions to Hempel's dilemma?

Question

SEP presents the dilemma as follows:

4.3 Hempel’s dilemma

One might object that any formulation of physicalism which utilizes the theory-based conception will be either trivial or false. Carl Hempel (cf. Hempel 1969, see also Crane and Mellor 1990) provided a classic formulation of this problem: if physicalism is defined via reference to contemporary physics, then it is false — after all, who thinks that contemporary physics is complete? — but if physicalism is defined via reference to a future or ideal physics, then it is trivial — after all, who can predict what a future physics contains? Perhaps, for example, it contains even mental items. The conclusion of the dilemma is that one has no clear concept of a physical property, or at least no concept that is clear enough to do the job that philosophers of mind want the physical to play.

One response to this objection is to take its first horn, and insist that, at least in certain respects contemporary physics really is complete or else that it is rational to believe that it is (cf. Smart 1978, Lewis 1994 and Melnyk 1997, 2003). There is an element of truth in this. It may be rational to believe that contemporary science is true, even if not that it is complete. Nevertheless, it also seems mistaken to define physicalism with respect to the physics that happens to be true in this world. The reason is that whether a physical theory is true or not is a function of the contingent facts; but whether a property is physical or not is not a function of the contingent facts. For example, consider medieval impetus physics. Medieval impetus physics is false (though of course it might not have been) and thus it is irrational to suppose it true. Nevertheless, the property of having impetus — the central property that objects have according to impetus physics — is a physical property, and a counterfactual world completely described by impetus physics would be a world in which physicalism is true. But it is hard to see how any of this could be right if physicalism were defined by reference to the physics that we have now or by the physics that happens to be true in our world. (For development of this point, and for a dilemma that is similar to Hempel’s but which casts the issue in modal rather than temporal terms, see Stoljar 2010; for discussion, see Baltimore 2013, Fiorese 2016)

A different response to Hempel’s dilemma is that what it shows, if it shows anything, is that a particular proposal about how to define a physical property — namely, via reference to physics at a particular stage of its development — is mistaken. But from this one can hardly conclude that we have no clear understanding of the concept at all. As we have seen, we have many concepts that we don’t know how to analyze. So the mere fact — if indeed it is a fact — that a certain style of analysis of the notion of the physical fails does not mean that there is no notion of the physical at all, still less that we don’t understand the notion.

One might object that, while these remarks are perfectly true, they nevertheless don’t speak to something that is right about Hempel’s dilemma, namely, that for the theory-conception to be complete one needs to know what type of theory a physical theory is. Perhaps one might appeal here to the fact that we have a number of paradigms of what a physical theory is: common sense physical theory, medieval impetus physics, Cartesian contact mechanics, Newtonian physics, and modern quantum physics. While it seems unlikely that there is any one factor that unifies this class of theories, perhaps there is a cluster of factors — a common or overlapping set of theoretical constructs, for example, or a shared methodology. If so, one might maintain that the notion of a physical theory is a Wittgensteinian family resemblance concept. However, whether this is enough to answer the question of what kind of theory a physical theory is remains to be seen. (For further discussion of Hempel’s dilemma, see the papers in Elpidorou 2018a.)

In short, either we define physicalism in terms of current physics, which is clearly imperfect and incomplete (e.g., The Crisis in String Theory is Worse Than You Think | Leonard Susskind), or we define physicalism in terms of a vague, idealized, hypothetical notion of physics that we will hopefully figure out in the future, but in that case we have no way to know in advance what such an idealized notion of physics will actually look like (e.g., perhaps it will include consciousness as fundamental, perhaps it will include spirits, etc.).

Is there any way out of this dilemma?

Answer 1

For people who reject the non-physical (i.e. physicalists), Hempel’s dilemma might not be a dilemma at all, and it's arguably shifting the burden of proof onto physicalists to define something they don't propose and don't accept (the non-physical, and how to differentiate that from the physical).

if physicalism is defined via reference to contemporary physics...
but if physicalism is defined via reference to a future or ideal physics...

I would say it's neither. It's a matter of what stuff is and what exists, not a matter of what we know or can know (although the two certainly go hand in hand, since we may or may not know what stuff is and what exists).

As per another answer of mine, one might consider the physical to be the place where all the things are that we can experience (directly or indirectly) with our basic senses - see, hear, smell, taste, touch (yes, this may be a bit fuzzy with proposed non-physical claims, as addressed in that answer). Physicalists don't see a reason to think there's any other place, and tend to reject claims that can't reasonably be argued to be in this place, in cases where we don't have empirical evidence for such things. This typically includes rejecting claims of gods and ghosts and souls and the afterlife and psychic powers and such (which tend to all be proposed to be outside of empirical investigation, to varying degrees).

Note, however:

• There may exist things in this place that we can never know, e.g. distant planets beyond our ability to ever sense, given the laws of physics. That wouldn't make those non-physical.
• This includes updating our knowledge as we gain more evidence. 1000 years ago, people may not have been justified in accepting the existence of quantum fields, because they lacked the supporting evidence. But now we have the evidence, so we're justified in accepting it. Physicalists wouldn't say any categorical switch happened there from non-physical to physical. If someone proposed it 1000 years ago, it would've still been a physical claim, just an unjustified one.
• This does not preclude someone proposing claims beyond empirical investigation, but it would place the burden on them to propose another means of investigation that would lead to accepting that claim, as well as providing a justification for why that means should be used (e.g. is it reliable?).

Hempel's dilemma presupposes that there is at least 1 more place or 1 more category of things. Otherwise it's asking physicalists to differentiate "the physical" from... nothing? Physicalists don't accept any additional categories beyond the physical, so it doesn't make much sense to ask them to draw a line between said categories, to explain what would fit into "the physical" and what would not.

In reality, the lack of clarity in what is meant by "physical" is more of a problem for those proposing that it is one of multiple categories of things. Physicalists just have that being everything that exists, so there's nothing to differentiate it from.

Another way of thinking about it

One might consider "physicalism" to be shorthand for: not accepting the existence or truth of gods, angels, demons, djinns, heaven, hell, valhalla, limbo, purgatory, the underworld, the astral plane, the thirteen heavens, baralku, barzakh, diyu, the spirit world, spirits, ghosts, telekinesis, transcendent souls, out of body experiences, "essences" of materials, levitation, clairvoyance, precognition, etc.

If one were to consider it as such, it would be much more obvious that Hempel's dilemma doesn't apply: there is no category, there is just a list of claims. Physicalists aren't the ones putting all those things into a category, other than the category of "things which we lack good reason to believe exist". People who accept those claims are typically the ones labelling it as non-physical, to try to explain why there is a lack of supporting evidence for those claims (without accepting that those things just don't exist). But of course one can also discuss any one of those things to consider whether there is or is not good reason to believe it exists.

But listing off all those items would take a while. It's far shorter to just label that "physicalism".

On that note (and as alluded to above), we can also somewhat consider physicalism to involve only accepting existence claims which are supported by empirical evidence (roughly speaking). But this would also entail rejecting the existence of proposed "physical" things that either don't exist or which we don't have enough evidence for yet - this is not a bad thing, but it might make physicalism less specific to the claims above, for which there might be some benefit to adopting the physicalism label in objection to (since many people believe such claims), to say that empirical investigation is really the only reliable means of investigation we have for existence claims.

Metaphysical or methodological?

There is a somewhat blurry line between physicalism as a metaphysical position (what is the nature of reality) versus a methodological position (how we approach evaluating claims).

We developed the methodology of using empirical evidence and rejecting things without empirical evidence. That leads to a metaphysical position where the nature of reality is said to just be this place we experience with our senses (existing in whatever form that it does), because we have no means of investigating anything that may be beyond that, and thus we have no reasonable way to conclude that such things exist.

Answer 2

You present the following as a creating a dilemma:

in that case we have no way to know in advance what such an idealized notion of physics will actually look like

One way out of the issue is to deny that the above quote is a problem and accept that we do not know how our conceptions of physics will evolve in the face of new observations as time goes by.

Answer 3

With the same reasoning we can say that the idea of fish is false. Our current understanding of fish is incomplete. There are many species still unknown to science. So until we have a full catalogue of all the fish in all the oceans, we have not a well-defined idea what a fish could be. So the idea of fish is a false idea. If we have catalogued all the species of fish, then the idea of fish is trivial. After all, who knows what kind of exotic fish we may find. We might even find out that humans and elephants are also fish after some kind..

And indeed: fish don't exist..

Answer 4

The rejoinder, "But other notions are difficult to analyze, yet we 'know them when we see them' regardless," doesn't seem entirely appropriate to this case. The analysis of the concept of knowledge, for example, is analysis of a relatively specifiable "thing," but the concept of the physical, per physicalism, is supposed to be indiscriminately all-encompassing—with the exception of things that happen to get the label "non-physical," even though the strict physicalist supposedly has to commit to the unintelligibility of this description/negation in the limit.

So a conception which we can't differentiate from its purported opposite or absence is either a weakly usable concept, or not stable, or whatever along those lines; or it is usable in some other way, but not for "present purposes." But since when does a physicalist have to actually deny the existence of any specific phenomenon? If theirs is, ultimately, a semantic non-discrimination perspective, wouldn't they be minded to stand at the midpoint between the horns of Hempel's dilemma? "Yes, the concept of physicality is open-ended and can absorb anything that science can absorb; this triviality underscores the falsity on the other side; yet then that is our intent, that we should not think that 'non-physical' phenomena are indescribable in the language of physics, but we should find a way to describe them in physicalistic terms—we are suggesting something almost normative about our use of language, that we should avoid trying to talk 'outside of' or 'apart from' our physically real language."

---

So you can also compare physicalism, to mysticism, say, in terms of epistemological "effects." Mysticism is abused when used to bolster overconfident assertions about various transcendent questions; but physicalism does subsume many robust samples of outright knowledge about the very large and the very small (and the dimensionless no-size-really-at-all). In this context, methodological naturalism, via the normativity of epistemology generally, expresses physicalism and its functionality, which is to be able to provide the kinds of powerful answers to powerful questions that the abuser of mysticism wishes to acquire by just wishing for it. The choice is then not between a trivial and a false ontology of physicalism, but between epistemic applications (including in ways that go beyond the trivial/false dichotomy: e.g. we will look for generic degenerate and nontrivial solutions to the given conceptual problems).