1. Fundamental physics and ontology

The Standard Model of particle physics posits entities you may have heard of such as quarks and gluons. Prior to its inception, the world was thought to consist of four fundamental forces:

• strong forces (the force that holds between, e.g., a proton and a neutron in an atomic nucleus)
• weak forces (the forces at play in radioactive decay)
• electromagnetic forces
• gravitational forces

The Standard Model unifies the first three.

The philosophical task is “to describe the ontology of fundamental physics in a way that is compatible with the notion that it has some causal or physical oomph.” (135)

Although Chakravartty is agnostic about nearly every aspect of this “oomph,” he notes one important commitment shared by all “traditional” theories of causation: that causation involves entities. Entities are broadly construed to include objects and events, construed in myriad ways.

This entity-based ontology stands in tension with structuralism, which frequently downplays the importance of entities in order to emphasize relations or structures.

2. “Particles”

Colloquially, the Standard Model is frequently thought to posit a variety of particles—protons, neutrons, electrons, bosons, mesons, etc.

However, these should not be thought of as point-like entities bouncing into each other. That would be to impose classical mechanics on the quantum world. In the Standard Model, “particles” are modes of excitation of a quantum field. 

Rather, “particle” is merely a placeholder for the properties associated with them (mass, charge, spin). This is an occasion for ontological theorizing, where various puzzles arise because quantum mechanics is weird. One of the biggest puzzles is permutation invariance:

• Swapping one particle for another of the same type within a system does not constitute a new physical arrangement. But if it is impossible to distinguish particles in this way, are they really distinct? So, it’s not even clear what we are claiming is real when we say, e.g., that electrons are real.

• 3. Structuralist interpretations

The standard (non-structuralist) picture holds that relations ontologically depend on their relata, the entities that stand in those relations.

• For instance, take the relation “is a student of”. If you and your professors (the relata of this relation) did not exist, then this relation would be empty. If you did some add/drop gymnastics such that you took courses with a different professor, the relation would change. (Notice the difference with fundamental particles: on the non-structuralist/standard picture, “is a student of” is permutation variant.)

Non-eliminative structuralism inverts the standard picture: entities ontologically depend on the relations in which they stand.

• Entities are just positions (locations, nodes) in patterns or structures. They have no intrinsic properties.
• It is opposed to particles having “kind identities.” While everyone agrees that different kinds of particles are individuated by mass, charge, and spin (among other properties), non-eliminative structuralists are unique in taking these to be extrinsic properties.

Eliminative structuralism goes even further: everything is a relation; nothing is an entity.

• Talk of entities is merely a convenient shorthand/useful fiction for referring to relations/structure.

Structuralism does a nice job with the Standard Model. If a particle can’t be individuated in a physical arrangement, then the most we can say is that it plays some role in that arrangement. But arrangements are just sets of relations, i.e., structures.

• 4. The Dilemma

Chakravartty presents a dilemma to both kinds of structuralism:

General schema	Eliminative structuralism	Non-eliminative structuralism
Given a version of structuralism, determine the locus of causal efficacy.	Concrete structures are the locus of causal efficacy.	Entities are the locus of causal efficacy.
In evaluating this view, encounter a metaphysical challenge.	How can structures be concrete if they have no relata? (cf. the standard picture.)	For this to be structuralist, entities must be defined entirely in terms of their extrinsic properties. But what kind of entity is that?
In replying to the challenge, posit a contentious ontological primitive.	Posit “relations-in-themselves” or concrete structures.	Posit “internally extrinsic entities.”
Dilemma: accept the contentious posit, or reject this version of structuralism.	Accept relations-in-themselves or reject eliminative structuralism.	Accept internally extrinsic entities or reject non-eliminative structuralism.

• 5. Dissolving the dilemma

Chakravartty uses the framework from Part I of the book to show how one can adopt different attitudes to different claims about subatomic particles. Most realists will endorse “that there are subatomic particles, conceived as entities subject to various detections, measurements, manipulations, and novel predictions” (159). There can be varying degrees of tentativeness in one’s attitude toward the more specific structuralist (eliminative and non-eliminative) proposals about particle ontology.

As you might have guessed, the varying degrees of tentativeness are a reflection of one’s epistemic risk, which in turn is determined by one’s stance. Chakravartty notes that suspension of belief is compatible with other kinds of epistemic attitudes. For instance, one might suspend belief about the structuralist ontology of particles, but still adopt “a pragmatic attitude toward theorizing about the finer-grained ontological natures of subatomic particles” that serves to “elaborate conceptual pictures of the ontology of fundamental physics in different ways, which may then be heuristically fruitful for scientific investigation down the road” (162).