The Problem of Physical Interaction
Reductive physicalism undermines itself and cannot explain how interaction happens at all. Let’s start with a syllogism:
All interaction is relational.
All relations are non-physical.
Therefore, all interaction is non-physical.
This is a valid argument; the conclusion necessarily follows from the premises. But are the premises true?
(1) should be uncontroversial. “Interaction” is inherently relational—an interaction is the state of A affecting the state of B.
Therefore, (2) is where the argument lies. I’m going to make the case that relations are, by definition, non-physical.
Atoms “Bumping Into Each Other”
Let’s take a simple example. Two hydrogen atoms colliding with each other.
No matter how you conceive of the atoms—as probability clouds, states of space, fields—the question arises: why don’t the atoms simply “pass through” each other? Why do they interact at all? Why do the states of one field/cloud/structure affect the states of another?
There are different ways to answer, but the answers look similar in the abstract. There are some physical objects and physical processes that give rise to other physical objects and processes, generating a regress of physical explanation. So at some point, physical interaction always ends up appealing to laws, rules, or principles—inherently abstract things that specify what happens when A relates to B in a particular way. It’s a variation of, “Specific structures behave in specific ways because of the laws of physics.” Those laws are sometimes captured with mathematical expressions.
Even describing the elementary “behavior” of physical objects requires abstractions—the concept of “behavior” itself is relational and non-physical, as “behavior” is about how patterns are related across time.
So “atoms bumping into each other” turns out to be a mixture of physical and abstract. Atoms are physical (let’s say), and bumping into each other is a highly complex, abstract, relational thing.
(Note: if you are tempted to say, “The relation is itself physical; it’s like a string attached to the two atoms”, that does not work, because we’d require an explanation for how the string affects the atoms—the string would be, in effect, another atom, and therefore we’d be left asking, “By what mechanism does the string interact with the atoms connected to it?”)
“It’s Just a Description!”
One attempt to avoid this conclusion is to say, “No no, physical laws don’t have any causal power. They are merely descriptions of observed patterns!”
But this only creates a bigger problem. The descriptionists are in principle rejecting explanations altogether, which is not far from a rejection of rationality. If rationality is about finding explanations for things, the descriptionist insists one should be content with no explanation for causal interaction at all.
Ironically, this position does give credence to the syllogism at the beginning of this article. If interaction is relational, and relations are non-physical, they conclude interaction must not be happening at all, and we’re left with the inexplicable appearance of interaction.
This leaves us at a fork in the road. When you look deeply enough at the world, it starts to look back, and the temptation is to close your eyes.
>But are the premises true?
2 is unsupported,t say the least . Spatial separation is clearly relational.. and clrarly physical, in that it objectively mrasureablr and quantifiable. The only objection would be the folk-physics objection that the physical is *stuff*.
>No matter how you conceive of the atoms—as probability clouds, states of space, fields—the question arises: why don’t the atoms simply “pass through” each other
If you conceive of it in terms of quantum mechanics, the answer is obvious: the pauli exclusion principle means that the electrons...which define the "size" of the atom, since the nucleus is tiny...cannot occupy the same state.. the electron orbitals either have to bounce off each other, or both atoms fuse into a new mutual state, a H2 molecule. No casual "passing through" is allowed.
Consider learning physics.
A quick note on the use of "reductive" in this substack before continuing. Whether if physicalism ends up being reductive, anti-inflationary, or fundamentally backwards depends on the default position one approaches physicalism from. If they are dualist then they'd give the reductive adjective. If they are already physicalist then they *might* give the anti-inflationary adjective to physicalism. If they are an idealist then they'd say physicalism is fundamentally backwards. If you haven't established what the correct default position is then it isn't necessarily the case that physicalism is reductive.
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Your "Atoms 'Bumping Into Each Other'" example for the regress problem presupposes a relata primitive ontology. That is, there are primitive stand alone substances titled atoms where relational interactions are derivative and between those atoms. What about a physicalism like Ontic Structural Realism though? An OSR like James Ladyman or Don Ross would argue relations are primitive and relata are emergent derivatives. This ontology for physicalism claims that reality fundamentally is an ontological co-determining web of relations where there are relational nodes. These relational nodes are more stable configurations of structural relations. Asymmetries in the stabilities of the co-determining relations web of reality are the objects interacting with each other with causality, space, and time. Objects, space, time, and causality would all be relational nodes in this ontology. This physicalist metaphysics better conforms to experimental outcomes found in quantum mechanics and general relativity.
A potential objection you might have is that a universal web of co-determining relations sounds circular. This is a category mistake. Circular reasoning is an epistemic concept which is dependent on a mind trying to justify beliefs. The universal web of co-determining relations is ontological and mind-independent.
If OSR is correct then every *thing* must go when dealing with ontological fundamentality. If interested in more details, I'd read "Every Thing Must Go" by James Ladyman and Don Ross. If you despise it then I'd love to read your attacks on it on substack!
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Scientific laws are descriptive and mathematically expressed. They are captured through the use of the scientific method on empirically observed regularities. Scientific theories are explanatory frameworks. The success of these theories in prediction and unification indicates that their mathematical structure mirrors the structure of reality itself. Scientific theories being explanatory within science means there is no risk of throwing away explanation and rationality by keeping scientific laws descriptive.