Composing Natural Systems

An interesting feature of Relational Quantum Mechanics (RQM) is its implication that discrete measurement-like interaction events are going on between natural systems (unobserved by us) all the time.  It turns out that this offers a way to incorporate quantum phenomena into an attractive account of how smaller natural systems causally compose larger ones.  In this post I will discuss the general approach, including a brief discussion of its implications for the ideas of reduction and emergence. In a follow-up post, I will discuss the quantum case in more detail with a focus on molecules.

Composite Causal Processes

The ontological framework I’m using (discussed in the last section of the prior post) is a modified version of Wesley Salmon’s causal process account (Salmon, 1984). The basic entities are called causal processes, and these comprise a network characterized by two dimensions of causation, called propagation and production. Propagation refers to the way an isolated causal process bears dispositions or propensities toward potential interactions with other processes--aka its disposition profile. Production refers to how these profiles are altered in causal interactions with each other (this is the mutual manifestation of the relevant dispositions).

The entities and properties described by science correspond to features of this causal web. For example, an electron corresponds to a causal process, and its properties describe its dispositions to produce change in interactions with other systems.

Given this picture, we can go on to form an account of how composite causal processes are formed.  What is exciting about the resulting view is that it can provide a framework for systems spanning the microscopic-macroscopic divide.

For background, I note that neither Salmon nor others who have explored causal process views provide a detailed account of composition. Recall that Salmon’s intent was to give a causal theory in service of underpinning scientific explanations.  In this context, he did outline a pertinent distinction between etiological explanations and constitutive explanations. Etiological explanations trace the relevant preceding processes and interactions leading up to a phenomenon. A constitutive explanation, on the other hand, is one that cites the interactions and processes that compose the phenomenon:

A constitutive explanation is thoroughly causal, but it does not explain particular facts or general regularities in terms of causal antecedents. The explanation shows, instead, that the fact-to-be-explained is constituted by underlying causal mechanisms. (Salmon, 1984, 270)

However, while Salmon sketches how one would divide a causal network into etiological and constitutive elements, he doesn’t provide a recipe for marking off the boundaries that define which processes/interactions are “internal” to what is to be explained by the constitutive explanation (see Salmon 1984, p. 275).

Going beyond Salmon, and drawing on the work of others, we can offer an account of composition for causal processes.  They key idea is to propose that a coherent structure at a higher scale arises from patterns of repeated interactions at a lower scale. We should pick out composite causal processes and their interactions by attending to such patterns at the lower scale.

In Herbert Simon’s discussion of complex systems, he notes that complexity often “takes the form of hierarchy (Simon, 1962, 468)” and notes the role interactions play in this context:

In hierarchic systems we can distinguish between interactions among subsystems, on the one hand, and the interactions within subsystems—that is, among the parts of those subsystems—on the other. (Simon, 1996, p.197, emphasis original)

The suggestion to take from this is that differential interaction rates give rise to a hierarchy of causal processes. When a group of processes interacts more with each other than with “outsiders” then it can form a composite. For example, a social group like a family or a business can be marked off from others (at a first approximation) by the differential intensity with which its members interact within vs. outside the group.

As part of his discussion of analyzing complex systems, Bill Wimsatt also explores the idea of decomposition based on interactions, i.e., breaking down a system into subsystems based on the relative strength of intra vs extra-system interactions. (Wimsatt, 2007, 184-6).  And while he describes how different theoretical concerns lead us to utilize a variety of analytical strategies, Wimsatt makes it clear that patterns of causal connections are the ultimate basis for understanding complex systems:

Ontologically, one could take the primary working matter of the world to be causal relationships, which are connected to one another in a variety of ways—and together make up patterns of causal networks…Under some conditions, these networks are organized into larger patterns that comprise levels of organization (Wimsatt, 2007, 200, emphasis original).[1]

Wimsatt explains that levels of organization are “compositional levels”, characterized by hierarchical part-whole relations (201). This notion of composition includes not just the idea of parts, but of parts engaged in certain patterns of causal interactions, consistent with the approach to composite causal processes suggested above.

To summarize: a composite causal process consists of two or more sub-processes (the constituting group) that interact with a greater frequency than each does with other processes.  Just like any causal process, a composite process carries its own disposition profile: here the pattern of interacting sub-processes accounts for how composite processes will themselves interact (what this means for the concepts of reduction and emergence will be discussed below). Consider social groups again, perhaps taking the example of smaller, pre-industrial societies. Each may have its own distinctive dispositions to mutually interact with other, similarly sized groups (e.g., to share a resource, trade, or to engage in raids or battle). These would be composed from the dispositions of their constituent members as they are shaped in the course of structured patterns of in-group interaction. We can also envision here that the higher scale environmental interactions also impact the evolution of the composite entity, but its stability is due to maintaining its characteristic higher-frequency internal processes.

Let me add a couple of further comments about composite processes.  First, as already indicated, a group of constituting sub-processes may be themselves composite, allowing for a nested hierarchy. Second, the impact of larger scale external interactions can vary.  Some may have negligible impact. Other interactions (especially if regular in nature) can contribute to shaping the ongoing nature of the composite. At the other extreme, there will be some external interactions that could disrupt or destroy it. The persistence of a composite would seem to require a certain robustness in the internal interaction pattern of its components. Achieving stability (and the associated ability to propagate a characteristic higher scale disposition profile) may require the differential between intra-process and extra-process interactions to be particularly high, or else there may need to have a particular pattern to the repeated interactions.  There will clearly be vague or boundary cases as well.

Why go to all this trouble of fairly abstract theorizing about a web of causal processes?  Because this account fleshes out the notions that underwrite the causal explanations scientists formulate in a variety of domains.

In the physical sciences, the familiar hierarchy of entities, including atoms, molecules, and condensed matter, all correspond to composite causal processes. Of course, in physical models, what marks out a composite system might be described in a number of ways (for example, in terms of the relative strength of forces or energy-minimizing equilibrium configurations).  But I argue this is consistent with the key being the relative frequency of recurring discrete interactions in-system vs. out-system. (This will be explored further in the companion post.)

In biology, the complexity of systems may sometimes defy the easy identification of the boundaries of composites. Also, a researcher’s explanatory aims will sometimes warrant taking different perspectives on phenomena. In these cases, scientists will describe theoretical entities that do not necessarily follow a simple quantitative accounting of intra-process vs. extra-process interactions. On the one hand, the case of a cell provides a pretty clear paradigm case meeting the definition of a composite process. On the other hand, many organisms and groups of organisms present difficult cases that have given rise to a rich debate in the literature regarding biological individuality. Still, a causal account of constitution is a useful starting point, as noted here by ElliottSober:

The individuality of organisms involves a distinction between self and other—between inside and outside. This distinction is defined by characteristic causal relations. Parts of the same organism influence each other in ways that differ from the way that outside entities influence the organism’s parts. (Sober, 1993, 150)

The way parts “influence each other”, of course, might involve considerations beyond a mere quantitative view of interactions, and connotes an entry point where theoretical concerns can create distance from the basic conception of the composite causal process. In a biological context, sub-processes and interactions related to survival and reproduction may, for example, receive disproportionate attention in creating boundaries around composite entities. Notably, Roberta Millstein has proposed a definition of a biological population based on just this kind of causal interaction-based concept (Millstein 2009).

It is also worth mentioning that constitutive explanations in science will rarely attempt to explain the entire entity. This would mean accounting all of its causal properties (aka its entire dispositional profile) in terms of its interacting sub-processes. It is more common for a scientific explanation to target one property corresponding to a behavior of interest (corresponding to one of many features of a disposition profile).

Reduction and Emergence

I want to make a few remarks about how this approach to composites sheds light on the topics of ontological reduction and emergence. In a nutshell, the causal composition model discussed here gives a straightforward account of these notions that sidesteps some common confusions and controversies, such as the “causal exclusion problem.”

When considering the relationship between phenomena characterized at larger scales and smaller ones, the key observation is that a larger entity’s properties do not only depend not only on the properties of smaller composing entities. They also depend on their pattern of interaction.  This is in contrast to the usual static framing that posits a metaphysical relationship (whether expressed in terms of composition or “realization”) between higher-level properties and lower-level properties at some instant of time.  This picture is conceptually confused (if taken seriously as opposed to a being a deliberate simplifying idealization): there is no reason to think such synchronic relationships characterize our world.

Recall that, in the present account, a property describes a regular feature of the disposition profile of a causal process. A composite causal process is made up of a pattern of interacting sub-processes.  The disposition profiles of the sub-processes are changing during these interactions: they are not static.  The dispositions of the composite depend on this matrix of changing sub-processes.  Note that both the forming of a higher-scale disposition (and its manifestation in a higher-scale interaction) takes more time than the equivalents at the smaller scale.  No composite entity or property exists at an instant:  this is a fiction concocted by us facilitate our understanding. Unfortunately, contemporary metaphysicians have taken this notion seriously.  It is perhaps easiest to see the problem in the case of a biological system:  nothing is literally “alive” at an instant of time.  Living things are sustained by temporally extended processes.  Less intuitively, the same is true of inanimate objects.

Emergence and reduction are clearer, unmysterious notions when based on this dynamic conception of the composition relationship.  Properties of larger things “emerge” from the interacting group of smaller things. The “reduction base” includes the interaction pattern of the components and their (changing) properties.  The exclusion problem says that since higher-level properties are realized by lower-level properties at any arbitrary instant of time, they cannot have causal force of their own (on pain of overdetermination). We can see why this is a pseudo-problem once a better understanding of composition is in place. Causal production occurs at multiple scales.

This take on reduction and emergence is obviously not unique to the causal process model discussed here.  It is implied by any approach that recognizes that properties of composites depend on interacting parts. For example, Wimsatt discusses at some length how notions of reduction and emergence should be understood given his understanding of complex systems. He offers a definition of reductive explanation that shows a similarity to the causal process view of constitutive explanation:

A reductive explanation of a behavior or a property of a system is one that shows it to be mechanistically explicable in terms of the properties of and interactions among the parts of the system. (Wimsatt, 207, 275)

This approach to reductive explanation is perfectly consistent with a form of emergence, in the sense that the properties of the whole are intuitively “more than the sum of its parts (277).” The key idea here, again, is that composition includes the interactions between the parts. For comparison, Wimsatt introduces the notion of “aggregativity”, where the properties of the whole are “mere” aggregates of the properties of its parts. For this to happen, “the system property would have to depend on the parts’ properties in a very strongly atomistic manner, under all physically possible decompositions (277-280)”. He analyzes the conditions needed for this to occur and concludes they are nearly never met outside of the case of conserved quantities in (idealized) physical theories.

Simon had introduced similar notions, describing hypothetical idealized systems where there are no interactions between parts as “decomposable,” which are then contrasted to “nearly decomposable systems, in which the interactions among the subsystems are weak but not negligible (Simon, 1996, 197, emphasis original).” To highlight this distinguishing feature, Simon considers a boundary case: that of gases. Ideal gases, which assume interactions between molecules are negligible, are, for Simon, decomposable systems. In the causal process account, we would similarly point out that an ideal gas doesn’t have a clearly defined constituting group: the molecules do not have a characteristic pattern of interacting with each other at any greater frequency than they do with the external system (the container). An actual, non-ideal gas, on the other hand, with weak but non-negligible interactions between constituent molecules, would correspond to the idea of a composite causal process.

Some contemporary work in metaphysics, focused on dispositions/powers and their role in causation, has incorporated similar views about composition and emergence. Rani Lill Anjum and Stephen Mumford describe a “dynamic view” of emergence:

 The idea is that emergent properties are sustained through the ongoing activity; that is, through the causal process of interaction of the parts. A static instantaneous constitution view wouldn't provide this (Anjum & Mumford 2017, 101)

In their view, higher scale properties are emergent because they depend on lower-level parts whose causal properties are undergoing transformation as they interact, consistent with the view discussed here.  Most recently, R. D. Ingthorsson's new book, while not discussing emergence and reduction explicitly, also presents a view of composition based on the causal interaction of parts which is in the same spirit (Ingthorsson, 2021, Ch. 6).

Conclusion

I think composite causal processes provide a good framework for understanding how natural systems are constituted.  A puzzle for the view, however, might arise via its use of patterns of discrete causal interactions to define composites. How would this work in physics, where the forces binding together composites, such as the Coulomb (electrostatic) force, are continuous?  One possible answer is to point out that physical models employ idealizations, and claim their depictions can still correspond to the “deeper” ontological picture of causal processes. But I believe we can find a better and more comprehensive answer than this.  To do so, we must look more carefully at physical accounts of nature’s building blocks, atoms and molecules, and see if we can uncover a correspondence with the causal theory. I think we can, assuming we utilize the RQM interpretation.  This is the subject of the next post.

References

Anjum, R., & Mumford, S. (2017). Emergence and Demergence. In M. Paolini Paoletti, & F. Orilia (Eds.), Philosophical and Scientific Perspectives on Downward Causation (pp. 92-109). New York: Routledge.

Ingthorsson, R.D. (2021). A Powerful Particulars View of Causation. New York: Routledge.

Millstein, R. L. (2009). Populations as Individuals. Biological Theory, 4(3), 267-273.

Salmon, W. (1984). Scientific Explanation and the Causal Structure of the World. Princeton: Princeton University Press.

Simon, H. (1962). The Architecture of Complexity. Proceedings of the American Philosophical Society, 106(6), 467-482.

Simon, H. A. (1996). The Sciences of the Artificial (3rd ed.). Cambridge, MA: MIT Press.

Sober, E. (1993). Philosophy of Biology. Boulder: Westview Press.

Wimsatt, W. C. (2007). Re-Engineering Philosophy for Limited Beings. Cambridge, Massachusetts: Harvard University Press.

Photo: Alina Grubnyak via Unsplash

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[1] This passage goes on to mention other, less neat, network patterns: “Under somewhat different conditions they yield the kinds of systematic slices across which I have called perspectives. Under some conditions they are so richly connected that neither perspectives nor levels seem to capture their organization, and for this condition, I have coined the term causal thickets (Wimsatt, 2007, 200).”

Russellian Monism and the Identity Theory of Properties

Here is a draft paper on a topic discussed a fair amount on this blog in the past.
Comments or suggestions are welcome.

Russellian Monism and the Identity Theory of Properties

[UPDATE: 6 Sept. 2013 - Very slightly revised from 2 Sept. version]

Here's the introduction:

Russellian Monism is an attractive approach to the mind/body problem. It promises to put both mental and physical phenomena on a common ontological ground. By providing a place in nature for the qualitative properties featured in conscious experience, it disarms prominent conceivability arguments against materialism. Russell’s approach can be strengthened by employing elements of a more contemporary metaphysical framework.  There is a particularly good fit with an account of the nature of properties set out by C.B. Martin and John Heil. Labeled the identity theory of properties, this view posits that properties are at once dispositional and qualitative.

This paper is organized as follows. In section one I offer an overview of Russell’s theory. In section two I briefly show how a key insight from Russell’s work has figured in contemporary debates in philosophy of mind. Section three takes a closer look at Russell’s metaphysics; this prepares the way for seeing how his theory might be modified in light of more recent work. Section four introduces the idea that the metaphysics of dispositional and categorical properties can play a role in a Russell-style account. Section five outlines the identity theory of properties and argues that its features can strengthen Russellian monism. In section six I consider objections to the modified theory, and discuss where it needs to be supplemented in order to more fully address the challenges of explaining mind.

 

 

Causal Constraint

The notion of dispositional modality, discussed by Stephen Mumford and Rani Lill Anjum in Getting Causes from Powers, put me in mind of another analysis of the interplay between causation and modality: that of Gregg Rosenberg in A Place For Consciousness (2004).

 Recall (see prior post here) that Mumford and Anjum analyze causation in terms of dispositions, or powers. These powers tend toward (dispose toward) their manifestations -- they do not necessitate them. Necessity is not the modality of causation. In addition, it is argued that dispositional modality is distinct from standard philosophical notions of possibility (logical or metaphysical possibility). Dispositional modality (dispositionality for short) does not involve “pure” possibility, since only certain manifestations are possible. In Chapter 8 of their book, the authors say: “Dispositionality…can be understood as a sort of selection function…that picks out a limited number of outcomes from all those that are merely possible.” Also: “The idea of a selection function is simply one that identifies a subset from a realm of possibilities. (p.189)”

In his book, Gregg Rosenberg introduced a model of causation which featured a notion which seems related to the idea of the selection function: this was a constraining function on the space of possibilities. Rosenberg, unhappy with both the Humean perspective on causation, as well as the theories of causal responsibility or causal production on offer, endeavored in chapter 9 of his book to strip down the notion of real causation to a bare minimum. This led him to the following notion of “causal significance”: “The causal significance of a thing is the constraint its existence adds to the space of possible ways the world could be…Causal significance shows causation to be an operator on a space of possibility. (p.150 emphasis original)” And: “It is a theory designed to understand how constraints propagate, so it explains how the actual world comes to be just a sliver of what could have been. (p.152)”

I say they seem like related notions, but constraint could be viewed as the negative image of selection. Selection picks out a few possibilities, while constraint rules out all of the others.

I find causal constraint to be a beguiling idea. Tentatively, it would seem to leave “pure” possibility in place as the fundamental metaphysical notion, in constrast with Mumford and Anjum’s argument for the irreducibility of dispositional modality.

Short Review of Getting Causes from Powers

In this book, Stephen Mumford and Rani Lill Anjum present their theory of causal dispositionalism, that is, causation based on dispositional properties, or powers. I highly recommend it to anyone interested in the philosophy of causation.

Powers do the causal work in our world, according to the authors: effects are brought about by powers manifesting themselves, and the manifestation is itself a further power or set of powers.

A central idea is that powers don't necessitate their manifestations - they dispose toward them. Causality has long been associated with the idea of necessity, and necessity (and the sense of constant conjunction) is too strong to describe causation. The main insight here is that other factors can prevent or interfere with the expected manifestation (and, indeed, they often do).

To help demonstrate how a disposition can be enhanced or, importantly, hindered by other powers, the authors develop a vector addition diagram. Only when the sum of vectors (with various strengths and directions) exceeds some threshold do we get the manifestation. They extend the model to more complex scenarios to argue that the model is robust enough to explain non-linear and even "emergent" behavior.

In addition to arguing strongly against necessity, the authors want to overthrow another usual notion. The authors reject as misguided the typical "two-event" conception of causation, where cause is temporally prior to effect, in part because no one has a compelling account of how you get from one to the other. Instead causes and effects are simultaneous - they are two aspects of a temporally extended process which brings about a change.

An important and creative part of the book explores the distinctive modality of dispositions in more depth. Dispositional modality (weaker than necessity but stronger than "pure" contingency) is the primitive and fundamental modality of nature. We derive necessity and possibility from our prior experience with dispositionality. Mumford and Anjum argue that we do indeed perceive causation, and present what they see as the clearest examples of this in the case of bodily sensation and specifically proprioception.

The book concludes with a compelling application: showing how the theory fits with processes studied in biology and genetics.

More on Causation and Reduction to Physics

I finished reading GettingCauses from Powers by Stephen Mumford and Rani Lill Anjum.  I recommend the book highly to anyone interested in causation, and I’ll be thinking about many of its arguments and themes for a long time to come.

As touched on at the end of my prior post, one possible challenge to models of causation, including the thesis of causal dispositionalism presented in this book, is the fact that causation doesn’t seem to comport well with physics.   The authors acknowledge this in their first chapter, referencing Russell’s discussion in his "On the Notion of Cause” (1913).  The issue is that dynamical equations associate states of a system with points in time, but nowhere do they invoke the idea of causal production.  They are symmetric with regard to time, where causation is not.  Mumford and Anjum respond in a couple of ways.  First, they say, the fact that causation doesn’t appear at the level of physics doesn’t mean it isn’t present at larger scales:  the reducibility of all phenomena to physics is a controversial idea which we are not compelled to accept.  We don’t know that physics represents a special fundamental level of reality in any case.  And given the provisional nature of scientific theories, should we let them trump our metaphysical reasoning?

This issue recurs as the book progresses.  In Chapter 4, the authors show how the composition of powers in causal situations can plausibly model emergent phenomena in the form of novel powers.  So the theory is robust if it does turn out that reduction of the phenomena in the special sciences isn’t possible.  And the final chapter of the book (ch.10) presents an interesting and persuasive application of the theory by showing how causal dispositionalism fits quite well with examples of processes studied in biology (including genetics).

Just like the situation in philosophy of mind, one must be cautious about drawing metaphysical conclusions from the perceived character of formal physical theory.

Determinism Doesn’t Imply Causal Necessity

A key part of the argument for causal dispositionalism in Mumford and Anjum’s Getting Causes FromPowers is the case against causal necessitarianism (chapter 3 in the book).   Causality is commonly thought to imply a necessary connection between cause and effect:  the authors say this is a mistake, and that the proper modality of causation is dispositional.   Causes dispose toward their effects - they don’t guarantee them.  The insight here is that other factors can prevent or interfere with the expected manifestation (and in everyday experience, they often do).   In fact, such prevention is always possible in causal situations, and if one moves to evade this fact by stipulating that prevention or interference is impossible, then the resulting necessity is not really coming from the causal process itself, but is being imposed in another way.

To see this, suppose I specify the causal factors involved in some manifested effect, and then someone points out another factor which could possibly interfere (despite my match being dry, a proper striking motion made and sufficient oxygen being present, a gust of wind might prevent the match from lighting).  Can’t I modify my scenario to specify that the threatening factor is absent (the wind is calm)?  Leaving aside the potential problem of listing an absence as a causal factor, the objector might present another possible interferer (a passing car might splash water on the match as it is being struck).  So, then I, in turn, specify that there is no nearby traffic, and so on.   In fact, no finite list of factors will ever suffice to rule out every interferer (however unlikely).  And by the time one is led to propose a “catch-all” condition, covering the whole state of the universe, we’re really not talking about a process of causal production anymore.

The authors note something interesting here.  They say that their argument against causal necessitarianism does not mean they are ruling out determinism.   This was a helpful observation for me because I have been guilty of confusion on this point.  One might think “determinism” means “causal determinism” which means “causal necessitarianism”.  However, determinism can be specified in other ways (including what might be the most common conception – see below).  Then causal dispositionalism could be compatible with determinism.  There is a causal process, and while it doesn’t necessitate effects, necessity is imposed in another way.

Note, that for the moment, we are leaving aside the idea of irreducibly probabilistic causation.  Such causation is likely a feature of our world (in fact I think the a posteriori case for it is nearly airtight), and therefore determinism is false.  But disentangling these ideas remains philosophically valuable.

As Mumford and Anjum say:  “The core idea in determinism is the fixity of the future by the past (p.75)” If one wanted to build a model of a deterministic world, causal necessitarianism is probably not the best tool, since the causal process doesn’t promise to cover all the possible loopholes - for instance if there are such things as uncaused events, then they would not be addressed.

It seems to me that the most common notion of determinism (probably inspired by classical mechanics) is this:  given a specification of all facts, and given comprehensive deterministic laws of nature, then the future is fixed.  There is no reason here to even mention causation – it adds nothing to the scenario.  One could be a Humean about causation and still endorse the deterministic picture.  And given the fact that in this physics-inspired vision the mathematical depiction of laws is symmetric with regard to time, it would be equally true to say that the past is fixed by the specification.  This is inconsistent with causation, which is not a symmetric process.  One might believe that the mathematically specified physical laws comprise a model of a causal world, but the laws themselves don’t constitute a theory of causation, and may very well be inconsistent with the idea of a causal process.

On this last point, I recalled a paper I had read a few years ago by Carl Hoefer: “Causality and Determinism: Tension, or OutrightConflict.”  In this paper, Hoefer defines a deterministic world specifically as one governed by deterministic micro-physical laws, and then goes on to argue that this definition is inconsistent with the presence of causation, using several philosophical theories from the literature as examples of how causation might be characterized.

A Note on Events and Causation

Presently I’m reading GettingCauses From Powers by Stephen Mumford and Rani Lill Anjum (and have finished six chapters out of ten).  I expect to blog more about this book, of which I think very highly.  I just wanted to very briefly comment on events, inspired by the treatment they are getting so far in the book.

Years ago, influenced by reading (later) Russell and Whitehead, I acquired the notion that (all else equal) there is an attraction to an ontology which gave a leading role to events rather than one primarily featuring substances (or objects) and their properties.  There seemed to be more potential for explaining the dynamic aspects of nature (including mind).

But while there has been an active modern debate on the nature of events, the most common depictions don’t seem to offer specific advantages to an event-focused ontology.  To greatly simplify, it seems philosophers would model events either as property exemplifications, in which case they are in danger of seeming much like static facts or states of affairs; or else events would be associated with spacetime locations, in which case they are little distinguished from objects, which are the quintessential occupiers of spacetime.  (The SEP article on events is here; an IEP article with additional focus on the theories of Kim, Davidson, and Lewis is here).  These sorts of models of events don’t seem to bring differentiated resources to metaphysical theorizing.

The goal of the Getting Causes From Powers book is to develop a theory of causation based on dispositional properties, or powers.  While powers play the leading role, their theory incorporates an intriguing view of events (at least causal events:  they don’t take a position on whether there are other sorts).  Specifically, causal events, which are manifestations of powers, are temporally extended processes.  The authors reject as misguided the typical “two-event” conception of causation, where cause is temporally prior to effect, in part because no one has a compelling account of how you get from one to the other.  Rather causes and effects are simultaneous – they are two aspects of a process which brings about a change.  Very Whiteheadian!

Do We Need Essences?

I’ve just been starting to read and think more about essences, in particular the debate which has followed Kit Fine’s argument in “Essence and Modality” (1993) that essences cannot be understood in modal terms.  The modal understanding is that an essential property of an object is one is must have of necessity (in order to be that object), while properties it can (possibly) do without are accidental.  Fine, on his way to advocating a definitional notion of essence, said the modal understanding was too broad:  an object may have certain necessary attributes which are intuitively not essential.  In the paper, he offered some examples intended to bolster this point.

Now I’ve read a few papers which take issue with Fine’s argument. In particular, one line of protest, due to Michael Della Rocca, notes that Fine’s examples are of necessities which seem trivially true of all objects or existents, and the modal understanding of essence can be recast by a focus on non-trivial necessary properties.

For present purposes, though, I want to concede that there is an intuitive sense that essence seems prior to its modal understanding (even if the latter turns out to be extensionally equivalent):  when I try to think of what properties are necessary to an object I seem to be appealing to some non-modal definition I have in mind.

But do we really want to add essences as irreducible elements in our ontology? While I’m attracted to some Aristotelian or neo-Aristotelian notions (such as causal powers), my inner Occam wants to resist essences.

One clue to a way to think about this dilemma occurred to me while reading Michael Gorman’s paper, “The Essential and the Accidental”.  Gorman highlights one of the passages about the nature of essential properties discussed by Fine in another paper (“Senses of Essence”, which I have not read):  “An essential property of an object is a constitutive part of the essence of that object if it is not had in virtue of being a consequence of some more basic essential properties of the object; and otherwise it is a consequential part of the essence.”  Perhaps this constitutive subset should be the real target of our idea of essence.

While Fine’s idea of distinguishing consequential properties from constitutive properties is one of logical consequence, Gorman takes this as an inspiration to develop an account of essence that depends on the notion of explanation. Perhaps the essential properties of an object are those which cannot be explained by appeal to other characteristics (while accidental properties are those that can be so explained).  The paper elucidates this argument and considers possible objections.  This view is distinct from the modal understanding because it can accomodate necessary but non-essential properties.

For myself, being in a very preliminary stage of studying these issues, I reserve my opinion about Gorman’s particular strategy, but am led to a desire to link essences to some other metaphysical problem, such as causation (which is obviously related to explanation).  By the time we conceive of an object, we already have in mind something which has been caused and has its own causal powers.  And we already know (I believe) that modality alone, say a mosaic of categorical properties distributed across possible worlds as in David Lewis, doesn’t provide a theory of causation.  So, it shouldn’t be a surprise if there is a problem with defining essence solely in modal terms if essence relates to causation.   I’ll try to see what’s been written along these lines.

UPDATE: 20 November 2011:
Kathrin Koslicki has posted a preprint of a chapter for a forthcoming book called "Essence, necessity, and explanation" which fleshes out a discussion similar to Gorman's.  She uses an analysis of Aristotelian notions of explanation, including cause, to account for how necessary but not essential properties follow from constitutive essential ones.  However, the scheme here is that essence is basic and prior to cause (I was wondering if there was a way to reverse this priority.)

Dispositional Modality as Restricted Possibility

I’ve been interested in the metaphysics of dispositional properties (or powers), and I’ve ordered Getting Causes from Powers, a new book from Stephen Mumford and Rani Lill Anjum. I look forward to reading this later in the fall, but it the meantime I have read a couple of related papers by the authors (see an earlier post here).

In “Dispositional Modality” Anjum and Mumford argue that the modal value of dispositions is distinct from necessity and possibility: it is described as “sui generis” and “irreducible”. What I thought was interesting, though, is that the authors themselves include a nice account of dispositional modality in terms of restricted possibility, which seemed to me had the flavor of a reduction.

I’ll pass over the first several sections of the paper, which covered some familiar ground: the fundamental disagreement with Hume about powers; the failure of the semantic reduction of dispositional ascriptions to conditionals; and the fact that dispositions, by their nature, clearly do not necessitate their manifestations (they are disposed toward, or tend toward their manifestations).

So, dispositional modality is distinct from necessity, but what about possibility? In section 5, Anjum and Mumford argue that dispositional modality is also distinct from possibility, in the sense that it is different from “pure” possibility. By pure possibility, the authors mean the broadest sort of logical or metaphysical possibility. A glass vase is disposed to shatter when dropped. One might suppose it is logically possible that the vase will turn to jelly when dropped, but it is clearly not disposed to do so – at least in our world.

The authors say we might think of dispositional modality as a subclass or restricted version of possibility. But this is already a familiar idea. Modal theorists talk about various nested classes of possibility: logical possibility (typically the broadest notion), various formulations of metaphysical possibility, and nomological possibility -- the subclass which holds the laws of nature fixed (some philosophers would identify one or more of these). Anjum and Mumford see dispositions as giving rise to “natural” possibility: “The reason some things are naturally possible is because there are dispositions for them.”

The authors don’t elaborate greatly on this point, but it seems as if natural possibility is the set of all the possibilities inherent in the dispositions contained in the state of the natural world at a given time. Building on this idea: when it comes to individual natural objects or systems, in saying they possess dispositional properties (or powers), we might equally well say they possess a certain restricted bundle of possibilities. Then we might turn to a discussion of how causation is explained in terms of the “actualization” of some of these possibilities (compare the “manifestation” of a disposition), perhaps depending on how a system interacts with other systems which similarly bear possibilities.

If one can retell the story of dispositions with restricted sets of possibilities, might this be a reductive analysis? It’s not completely clear to me, because one might say the introduction of restricted sets of possibility of the type needed is an irreducible extension of our notions of modality. And perhaps the idea of an object bearing a set of possibilities, rather than bearing properties, isn’t coherent (but maybe the terminology can be worked out). In any case, this paper’s comparison of dispositional modality to the idea of restricted possibility was very thought-provoking.

More on the Problem of Being for Powers

This is a brief follow up to the discussion at the end of the last post. The problem of being for pure powers is the challenge of explaining their nature or ontological grounding when not manifested (if they are not grounded by other properties). I had been exploring the model that powers have the ontological character of quantum systems between measurements (=manifestations). And I think the best way to evaluate the being of quantum systems is as truly existing but non-concrete propensities (while manifestation events=concreta). I hesitate to use the term “abstract” to describe this sort of being, since that has come to imply “causally irrelevant”, and powers are anything but – indeed they underwrite causality in nature. But I do think we need to accept that there are indeed two modes of being.

I draw connections to two papers recently discussed.

I think William A. Bauer’s notion that pure powers are self-grounding due to ongoing minimally sufficient manifestations can connect with the above view in an interesting way. Our analysis of powers typically begins with paradigm cases involving everyday macroscopic objects (like the fragility of a vase). Well, the strange ontological status of quantum systems is only apparent when very small systems are considered in isolation. In everyday situations larger systems do have ongoing minimal interactions with the environment – leading to the kind of apparent sustained being that Bauer envisions. So while his explanation for the self-grounding of powers is not ultimately correct, it connotes an approximate fact about the ongoing manifestation of powers in the rough and tumble world.

Finally, I would note another interesting connection between the view I’ve sketched lining up powers with quantum systems by referring back to the Neil E. Williams paper I discussed in my post "Power Holism". Williams wondered how multiple powers had the capability of fitting together to produce mutual manifestations, arguing that one may need to appeal to a kind of holism to explain this. I would just quickly note that the ability of quantum systems to become correlated through non-local connections and entanglement provides a model of holism which may address his concerns.

Power Property Papers Perused

Here are some brief thoughts on papers I read recently on the metaphysics of powers/dispositions.

I thought “A Powerful Theory of Causation” by Rani Lill Anjum and Stephen Mumford was an important paper. The authors set out to show that powers have the right degree of “modal strength” to support a theory of causation. Powers have been long thought to necessitate their manifestations, and necessity (and the sense of constant conjunction) is too strong to describe causation. The notion of metaphysically necessary connections in nature has long supplied a basis for arguing against powers and associated theories of causation. Anjum and Mumford say that powers (dispositions) “dispose” toward their manifestations, but don’t necessitate them.

One way of seeing that powers fall short of necessity is to note that when placed in a context, a disposition can be enhanced or, importantly, hindered by other powers. The authors use a vector addition model as a heuristic to see how this works. Only when the sum of vectors (with various strengths and directions) exceeds some threshold do we get the manifestation.

In later sections of the paper the authors deal with various potential objections and place their theory in a historical context of the difficulties faced by causal models, showing again that the unwarranted assumption of necessitation was the key stumbling block.

While the vector model was interesting, my favorite section of the paper (section 5) deals with explaining probabilistic causation. Here Anjum and Mumford endorse a propensity (propensity=probabilistic power) interpretation for a single disposition. I myself think this is the key to understanding how powers can have the right modal strength “all the way down”; it also has the virtue of fitting with our best physical theory of how the actual world works (quantum mechanics).

I eagerly look forward to a forthcoming book from Mumford and Anjum called Getting Causes from Powers – this will elaborate upon the theory in greater detail. Also note that a podcast and slides from a recent talk by Stephen Mumford from the PhilSci forum at UMB (Norwegian University of Life Sciences) are available here (scroll down for previous talks). It is a very nice introduction to powers, and focuses on contrasting a powers approach with a laws-based theory of causation.

Some quick takes on other papers.

Michael Esfeld argues that a metaphysics of powers has an advantage in terms of compatibility with physics in his paper “Humean metaphysics versus a metaphysics of powers.” In the paper, Esfeld summarizes the difference between a Humean approach and the powers approach: in contrast to the above he does characterize powers as having necessary connections with their manifestations. However, he does then describe the option of treating powers as propensities to explain probabilistic causation. His main point in the paper is that while physics can be compatible with more than one metaphysical picture, the powers model is the best fit given the commitment of physics (and other sciences) to describing dispositional and functional properties.

William A. Bauer takes up the issue of the ontological grounding of powers in "Four theories of pure dispositions". Can they exist without depending on categorical properties for their being? What is the nature of powers when they aren’t manifesting if they lack such grounding? He reviews several approaches to this problem, and concludes that the best model is one where they are self-grounding via a continuous low-level manifestation (which is distinct from their more pronounced, distinguishing potential manifestation).

This was a thought provoking paper, which prompted me to go back and review others on this topic, including Mumford’s “The ungrounded argument”, Neil E. Williams’s response “The ungrounded argument is unfounded”, and Stathis Psillos’s 2006 paper “What do powers do when they are not manifested?”. My quick two cents on this issue is inspired by QM and the idea of powers as propensities: I think powers have a real-but-not-concrete status akin to possibilities in a framework of modal realism. Unlike a static notion of possibilia, however, propensities causally impact our world through their disposition toward actual manifestation events. This is again consistent with QM: between measurement events, quantum systems don’t have concrete existence, but they certainly exist in a causally relevant way, as their influence on events is apparent and measurable.

Finally, I note with interest that two philosophers have recent papers endorsing the Heil/Martin view of powers as inherently qualitative: Jonathan D. Jacobs in “Powerful qualities, not pure powers”, and Robert Schroer, in “How far can the physical sciences reach?”. Both make for interesting reading, as do other papers by these authors.

Here is a list of prior posts on powers/dispositions (chronological).

George Molnar and the Powers That Be

Pan-Intentionality

Powers and Property Dualism

Modal Problems with the Theory of Powers

John Heil Gets Very Close

Suarez on Quantum Propensities

Notes on C.B.Martin's The Mind in Nature

Power Holism

Powers vs. Humean Supervenience

Abilities vs. Dispositions

Russellian Monism and Dispositional/Categorical Properties

{Note: this is a draft of some work that I might develop further with add'l research at some point. Comments or suggestions are welcome.}

I’m gratified that the position in Philosophy of Mind known as Russellian Monism (also known as Russellian theory of mind and probably the best developed account of neutral monism) has gotten more attention in recent years. However, the terminology typically used to describe the position today is different from Bertrand Russell’s, as presented in his 1927 work, The Analysis of Matter. This post discusses some of the issues involved, and briefly looks at how some stances in contemporary debates would fit with the original account.

In a recent post on the Brains blog, Richard Brown (referencing an online discussion he had with David Chalmers) said: “RM [Russellian Monism] is the view that the dispositional properties talked about by physics have as their categorical base phenomenal or protophenomenal properties.” While descriptions vary, the reference to dispositional and categorical properties is common. In his book, Ignorance and Imagination, Daniel Stoljar says the position is a combination of two theses. First: “…that physical theory tells us only about dispositional properties.” And: “The second thesis we need to consider is that the dispositional properties of physical objects do require categorical grounds; that is, for all dispositional properties, there must be a non-dispositional property... (p.110)”

Now, Russell never uses discusses properties at all, and certainly not dispositional or categorical properties specifically! These are terms which have emerged in the more recent debates of analytic philosophy. So, how well is the intent of RM captured when using this terminology? (Please again note I’m only speaking of Russell’s work in The Analysis of Matter).

Brief summary of RM

Abilities vs. Dispositions

John Maier (of ANU) does research on the philosophy of abilities. I read his SEP article on abilities, and a draft paper entitled “An Agential Theory of Dispositions.” In the latter, he argues that dispositional properties (also known as dispositions, and sometimes powers) can be analyzed in terms of agents' abilities. The idea is that claims about dispositions in the world are a “projection of agent-centric facts about manipulability onto a world whose nature, considered in itself, is exhausted by the categorical.” Since I have been interested in ontological accounts which place dispositions in a central, fundamental role (see list of posts below), I was interested to see Maier’s argument for this alternative proposal.

Powers vs. Humean Supervenience

I’ve been interested in ontological accounts which feature dispositional properties (also known as dispositions or powers) in the starring role.  This intriguing draft paper, “Goodbye, Humean Supervenience”, by Oxford's Troy Cross, sets out to show that David Lewis’ Humean supervenience program (which seeks to reduce everything to a mosaic of categorical properties) fails, since it can be reinterpreted as a system where Lewis’ fundamental properties are in fact identical to dispositions.  This implication can’t be rejected without abandoning the account of properties as sets of possibilia.   The possibility unattractive feature of the dispositional doppelganger of Lewis’ story is its appeal to nonlocal circumstances as the trigger for the manifestation of the powers.  My notes on the paper follow.

2 Ontology Papers

1. I discovered on the web the writings of Ian Thompson. He is a physicist by career but someone who is also well versed in philosophy and has been setting out his own stance on things metaphysical. In “Power and Substance” he says some things I thought made a lot of sense. He likes the idea that dispositional properties (or powers) are ontologically essential. He sets out an argument that dispositions could be taken to constitute a substance rather than as properties. I am intrigued by this argument, but agnostic about it on first reading. What I really liked (hopefully not just because I agree) is his take on how a dispositions-based ontology comports with the picture of quantum physics (section 6).

Power Holism

This makes a nice follow up to my recent reading of C.B. Martin’s book: I found this paper, “Puzzling Powers: The Problem of Fit” by Neil E. Williams. In it, Williams identifies and seeks to address a trouble spot encountered by Martin and other advocates of a powers/dispositions-based ontology. To start, Williams describes three key features of the ontology. First: the powers are intrinsic properties of their bearers. They don’t need external or relational connections for support. Second, the manifestations which they are capable of producing are essential features of the power: those potential manifestations make a power what it is. Third, the actual manifestations occur as a result of reciprocity. Martin in particular stressed the mutual nature of powers working together to produce manifestations, noting different pairings or combinations of powers will lead to different outcomes. Given these three features, Williams sees “a problem of fit.” He says: “Stated briefly, the problem is that powers have to work together when they produce manifestations (reciprocity), but as they are not relations (intrinsicality), and they cannot change with the circumstances (essentialism), the fact that they are causally harmonious is without explanation.” Powers fit together to produce mutual manifestation, but the fact that they fit is not accounted for by the three features they possess. He uses a jigsaw puzzle analogy to suggest that, as it stands, there’s no reason to expect a fit to occur without adding more to the story. So, what to do? Williams doesn’t think it’s advantageous to drop one of the three features he began with. With regard to essentialism, he does mention an alternate idea that the potential manifestations essential to powers are not determinate but are instead “TBD” (to-be-determined) via hooking up with other powers. But he still sees a gap with this idea: what finally makes the particular determinate manifestation finally occur? So, instead, Williams puts forward a solution: “power holism”. The nature of powers is determined holistically: “the specific, determinate nature of each power (that is, the set of manifestations a power is for and the precise partners required for those manifestations) depends on the specific, determinate nature of other powers with which it is arranged in a system of powers.” He notes we also need to discuss the bigger picture of “…what kind of world allows for or provides for the fit that power holism bestows.” How do the powers get their holism on, in other words? He looks at three ways one might address this. First, we could just posit holistic coordination as a brute addition to ontology. But this isn’t very satisfying. Second, he suggests one might look to a platonic account: collaboration between powers takes place in the platonic realm. Third, one might retain naturalism, but suggest a form of monism which could support the coordinated fit. For instance, if all powers ultimately ontologically depended on the prior existence of the whole world (a la Jonathan Schaffer), then this shared basis could explain the harmonious fit. Williams is sympathetic to this option but says any of the choices might be worthy of consideration. A very good paper. I liked the appeal to holism and (although Williams doesn’t use this terminology) non-local connections. In my opinion, though, in order to really nail it down, some indeterminism needs to be added to the model. Fit (correlations) can be explained by the non-local connections, but powers need to be seen as capable of more than one outcome per partner—an irreducible indeterminism only resolved when the manifestation (event) occurs. This would then nicely comport with quantum mechanics. (I see Williams also has a paper co-authored with Andrea Borghini advocating a single-world modal actualism, explained using a powers ontology. I’ll have to check that out – my last post on that topic was here).

Notes on C.B.Martin’s The Mind in Nature

Philosopher C.B.Martin died last year and left us a great book. The Mind in Nature summarizes his philosophy and its applications to mind, causality and more. For background see Paul Snowdon’s obituary and a brief note on the book by Gualtiero Piccinini; here is a very good draft review by Jessica Wilson. It was not an easy book for me to read but I found it very rewarding. Martin proposes an ontology featuring dispositions (sometimes referred to as powers). Notably for him, dispositions are inherently qualitative, and are also capable of producing myriad manifestations depending on the context.

I have previously read the work of philosophers who feature dispositional properties/powers as a basic element in their ontologies. (Martin has an early section recounting arguments for the irreducibility of dispositions– see the Wilson review for more on this). An attraction of these proposals is their potential to support a theory of real causality and of intentionality – topics mainstream analytic philosophy has trouble with IMO (see my series on George Molnar for more). John Heil adds the twist (along with Martin) that we could consider dispositions to also be qualities, which helps solve an additional aspect of the mind/body problem. Despite these accomplishments I have continued to view disposition-based accounts as falling a bit short when it came to mind and also to modality (see this recent post). Martin’s distinctive account takes another step toward addressing these issues.

Dispositions Offer Possibilities

I think the key to Martin’s ontology is that his dispositions don’t just point to one kind of manifestation, they are prolific. He says that whatever the fundamental elements of nature are, they have multiple internal properties which are dispositions to potentially an infinite number of different manifestations depending on context. Different “reciprocal disposition partners” will give you different manifestations. At any point, what exists is a “…manifestation tip of a disposition iceberg (p.9)”.

One doesn’t need “possible worlds”, since dispositions and their “projectivity” of actual and non-actual manifestations with various partners constitute a web or “power-net (p.29)”. In fact actual readinesses exist for infinite potential (Wilson points out this assertion is not backed up by an argument), so you don’t need any further grounding for possibility. (Upon reading this my own thought was why couldn’t Martin just redefine possible worlds as possible power-nets?). I note Martin himself isn’t vehement in asserting he’s definitely provided a full and adequate grounding for modality, compared to his tone when defending other aspects of his view.

Emergence/Reduction

For Martin, the creative power of dispositions means you don’t need “levels of reality” or the notion of supervenience. He sees all reality as a continuum “going from the many directional readiness of the quark, most of which will never be manifested, to the capacities and dispositions for many representations of some English speaker, most of which also will never be manifested (p.29)”. (Note that in contrast to the mainstream of analytic philosophy, language is not primary: the ontology comes first). The basic ontology is rich enough that emergence occurs through processes at the one level of reality; emergence is not a concept implying or requiring multiple levels.

Causality and Events

The manifestation of a disposition is the basis unit of causality. At one point (Ch.5), Martin stresses that causality in his view is not a temporally separated thing. The reciprocal partnering of dispositions creates a mutual manifestation; the model is of a “cause-effect” rather than cause-then-effect. (I would say, in other words, an event.) Martin doesn’t much address the problem of time and its perceived unidirectional flow. He takes the space-time of Einstein to be a substratum carrying the dispositional properties.

Mind

With regard to mind, Martin stresses that dispositions (which are inherently directional) give you intentionality at level below what we think of as “mental”. He also gives an account of how a natural system utilizing representation comes into being; he is inspired by results in neurobiology which he takes to show that “vegetative” (i.e. unconscious) systems of the brain effectively utilize representations already. So intentionality and representation are not distinctive hallmarks of human consciousness.

And neither are qualities. Martin first says dispositions cannot be identified with structural properties (such a view leads to an empty “Pythagoreanism” in his view). Even structureless elementary units (quarks or whatever) have multiple internal dispositions. And there is no need for purely qualitative, non-dispositional (categorical) properties. It is dispositions through-and-through and these dispositions are themselves simultaneously qualities (Wilson’s review also discusses criticism of this dual-aspect idea citing Armstrong).

So what does distinguish the mental? Here Martin offers a model which says the difference between mental and non-mental lies in the kind of qualitative material used in a representation. If the material of use is appropriately sensory, we get consciousness. I’m giving this model short-shrift here a bit in my notes (chs.13 -15), but I was a bit disappointed by it. Given Martin’s emphasis on gradualism, i.e. having nature exploit its base level qualities to give rise to all phenomena, I found his account of mind to be ad hoc. Since we’re making some brute assumptions in our ontology anyway, I would prefer to just couple the experiential aspect of reality to the qualitative aspect at the base level as two views of the same thing (possessed by each mutual manifestation). Then the emergence of human consciousness would be likewise gradual and not wholly dependent on functional criteria. (Wilson has a somewhat similar critique in the last part of her review).

Propensities and Quantum Physics

I was happy to see Martin tries to grapple (briefly) with quantum physics (in his chapter 6). In a section called “Dispositions and Quantum Theory”, Martin briefly discusses quantum theory and the interpretational question of assigning ontological status to the wave function vs. the measurement events. He thinks a system as described by the wave function could be taken as a propensity, which is a bit different idea than a disposition which points to specific manifestation. There seems to be a metaphysical gap here (correlating to the problem of the “collapse of the wave function”). He suggests that a “dispositional flutter” or oscillation might cause the appearance of irreducible probability. He says the flutter could be a consequence of practical limits on detection, or an intrinsic random oscillation. Unfortunately, I think we know this kind of interpretation of QM doesn’t hold up.

I thought it would have been consistent with his views if he identified his dispositions as wave function-type propensities and than had his “mutual manifestation” event map to the quantum measurement event. There still is an element of apparent mystery or gap surrounding the triggering of events, but I think we have to accept that this is just a given feature of the world, given QM.

Suárez on Quantum Propensities

Interpreting dispositional or power properties as propensities seems to me to be a very promising avenue for ontology. This is because theories employing powers (the ones I’ve seen) don’t get the modal structure of the world correct: by assuming that powers entail their manifestations, they fail to provide truthmakers for possibilities. Taking powers to be probabilistically manifested propensities solves this problem. It also bolsters a realist account of causality: actualizing propensities into specific outcomes gives causation some “real work” to do.

Finally, propensities can serve as a link between a philosopher’s ontology and the interpretation of quantum mechanics.* I was happy to find recently (via Philpapers) that propensities have a champion in Mauricio Suárez, a philosopher of science at Complutense University of Madrid. In several papers he has explored and advocated the propensity approach to understanding the properties of quantum systems. He also has pointed out the value of propensities to the dispositional/power property approach to ontology.

Popper and propensities

Propensity theory seems to be a relatively neglected topic these days. The work of Karl Popper may be one reason for this. Given Popper’s stature, the fact that his propensity interpretation of probability is widely regarded as a failure is discouraging. But Suárez makes a strong case that the problems with Popper’s theory are the result of his emphasis on interpreting quantum probabilities specifically, and also due to some particular assumptions which can be set aside or modified.

Popper wanted to interpret quantum probabilities using propensities, and he also thought propensities could be used to interpret probability in an objective manner generally. This effort has been roundly criticized. An important criticism is that known as Humphreys' paradox (after Paul Humphreys, see this paper). Humphrey pointed out that the asymmetric causal nature of propensities made them inconsistent with the symmetric character of conditional probability. But this paradox is only a problem for propensity interpretations of probability. When it comes to quantum theory, Suárez makes a wise point when he says that what we want to do is interpret quantum mechanics, not quantum probabilities. The probabilities observed in experiments would be explained by our account of quantum mechanics. This “clicked” for me: in my own reading of papers on interpreting quantum probability, I had found that the arguments tended to point toward subjective or Bayesian interpretations (see posts here and here), but this work didn’t seem to help one progress toward a satisfactory ontological interpretation of the physics. Perhaps it is better to interpret the ontology first.

With regard to some of Popper’s other assumptions about the nature of quantum propensities, Suárez explains in the paper “On Quantum Propensities: Two Arguments Revisited” how two other criticisms of Popper’s view may be avoided by a revised account of propensities – specifically Suárez’ ”selective propensities” proposal.

Selective Propensities

In addition to the paper mentioned above, Suárez has two other papers which discuss his selective propensities approach. In “Quantum Propensities”, he looks at some other historical attempts to employ propensities to interpret QM, and then contrasts his own proposal. 2004’s “Quantum Selections, Propensities, and the Problem of Measurement” develops the approach in the most detail, showing how it builds on Arthur Fine’s “selective interactions” solution to the quantum measurement problem.

Suárez’ approach is new to me and I’m still trying to understand it (I had not been exposed to Fine’s work before either). It seems that in the selective propensity interpretation, a quantum system possesses a number of dispositional properties coinciding with the observables we measure in experiments involving particles. These properties manifest themselves consistent with the probability distributions we observe in QM. We assert that in a measurement one interacts only with the property of the system selected. The interpretation then says that to explain the result, we can employ a mixed state of that property’s eigenstates to describe the initial preparation, rather than plugging in the full quantum state of the system. (The full quantum state encompasses all of the system’s properties.) We can still interpret the interference effects which result in some experimental setups as due to the interplay among the system’s various properties consistent with the full state superposition of the system.

This seems to imply that the description of the initial state of the system (setting up either a mixed state over one observable or the full state) is altered by how we set up the experiment. This seems strange at first glance, but I guess there’s always going to be something strange when you’re working with QM. I also wonder how to think about generalizing this scheme to understand how interactions work beyond the laboratory setting.

I’ll try follow up with more after re-reading and digesting this material further.
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* I’ve often thought about the issues involved when philosophers try to make sure their metaphysical ideas comport with physical theories. On the one hand, philosophers very much want to avoid proposals which seem to conflict with science. On the other hand, since our physical theories are provisional (and likely to be replaced in time by improved theories), perhaps philosophers shouldn’t worry if well-motivated ideas imply revision to current scientific understanding. I’ve seen relativity theory invoked to criticize philosophical positions (e.g. presentism in the discussion of time – see an abstract of what looks like an interesting paper here), but many recent research programs in quantum gravity explore the idea that relativity is an effective (low-energy regime) theory rather than something fundamental. The search for a theory of quantum gravity implies relativity, quantum mechanics or both will need to be revised.

So, while I personally want my metaphysical theory to accommodate quantum mechanics (and worry less about conflicts with relativity), I realize that this is tricky territory. It seems best to just be explicit about one’s presumptions.