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In Defence of Chalmers’s Positive-Fact Formulation of Physicalism

This paper defends Chalmers's positive-fact formulation of physicalism against Luenenburger's objection regarding blockers and two other criticisms, ultimately arguing that this approach offers a simple and defensible account of physicalism.

Original authors: Meysam Vahedi

Published 2026-10-16

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Meysam Vahedi

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

At the heart of modern science lies a simple, powerful idea: everything that exists is made of physical stuff. This view, known as physicalism, suggests that if you could list every single physical fact about the universe—the position of every atom, the strength of every force, the flow of every energy—you would have a complete description of reality. There is no room for ghosts, souls, or magical forces floating outside this physical machinery. However, philosophers have long struggled to write down a perfect definition of this idea. The trouble begins with a logical snag: if you say that the physical world determines everything else, you run into a paradox where you accidentally prove that things like ghosts are physically impossible, even in worlds that look exactly like ours. This creates a dilemma for scientists and thinkers who want to keep their definition of reality both strict and sensible. They need a way to say that our world is purely physical without accidentally claiming that the laws of physics forbid ghosts in every possible universe, a claim that feels too strong and unnecessary.

A recent paper by Meysam Vahedi tackles this long-standing puzzle by defending a specific solution proposed by the philosopher David Chalmers. The paper enters a debate between two main ways of fixing the definition of physicalism. One approach, championed by Frank Jackson, tries to solve the problem by adding a "stop clause" to the definition, essentially saying that a physical world must contain nothing more than what is strictly required by its physical laws. The other approach, Chalmers's, suggests we should only look at "positive facts." In plain terms, a positive fact is something that actually exists or happens, like a tree growing or a person feeling pain. A negative fact is the absence of something, like the fact that there are no unicorns in the room. Chalmers argued that a physical world is one where all the positive facts are determined by the physical, while the negative facts (like the absence of ghosts) do not need to be determined in the same rigid way. This distinction allows the definition to work without making impossible claims about other universes.

Vahedi's paper argues that Chalmers's approach is not only correct but superior to the competing "stop clause" method. The author addresses several criticisms that have been raised against Chalmers's idea. One major objection comes from the concept of "blockers." Imagine a scenario where the physical laws of a world are perfectly set up to create a mind, but a mysterious, non-physical force steps in to stop it from happening. Critics have argued that Chalmers's definition would wrongly say this world is not physical, while the other definition would say it is. Vahedi counters this by arguing that a true physical world should not allow for such blockers in the first place. If the physical facts are truly sufficient to create the mind, then nothing external should be able to stop them. Therefore, the definition that rejects these "blocker" worlds is actually the more accurate one, and Chalmers's formulation handles this correctly.

The paper also rebuts the claim that the original problem of "extras" (the ghost paradox) was never a real problem to begin with. Some critics suggested that we don't need to worry about negative facts like the absence of ghosts because they are obvious. Vahedi explains that this view misunderstands how physicalism works. Just because we believe ghosts don't exist doesn't mean the physical facts alone prove it without any extra philosophical reasoning. The definition of physicalism must be precise enough to handle these absences without breaking. Furthermore, the author tackles a complex logical objection regarding how we compare different possible worlds. Critics claimed that Chalmers's idea of "positive facts" leads to a contradiction when we imagine adding a blocker to a world. Vahedi resolves this by clarifying how we should think about one world being a "part" of another. He argues that if you add a blocker to a world, you are fundamentally changing the nature of that world, so it is no longer a simple extension of the original. This means the logical contradiction disappears, and the definition remains consistent.

Ultimately, the paper concludes that defining physicalism in terms of positive facts provides the clearest and most robust account of reality. It avoids the logical traps that plague simpler definitions and stands up better against complex philosophical challenges than its main rival. By focusing on what actually exists rather than what does not, Chalmers's formulation offers a way to understand the universe as entirely physical without getting tangled in impossible scenarios. The research does not claim to have discovered a new physical law or solved the mystery of consciousness itself; rather, it refines the map we use to navigate the relationship between the physical world and everything else. It suggests that the best way to describe a physical universe is to say that everything that is actually there is determined by the physical, leaving the question of what is not there to be handled with a bit more flexibility. This careful adjustment helps keep the scientific worldview coherent, ensuring that our definition of reality is as solid as the facts it describes.

Technical Summary: In Defence of Chalmers's Positive-Fact Formulation of Physicalism

Problem Statement
The paper addresses the "Problem of Extras," a significant challenge to standard supervenience-based formulations of physicalism. The standard formulation posits that physicalism is true of the actual world ( $\alpha$ ) if and only if every fact in $\alpha$ supervenes on the physical facts of $\alpha$ . Critics argue this formulation is too demanding: it implies that the non-existence of non-physical entities (e.g., Cartesian souls) is physically necessary. Consequently, any possible world physically identical to $\alpha$ must also lack these entities. This renders the absence of extras a physical necessity, a claim many physicalists wish to avoid, as it suggests that physicalism entails the physical impossibility of dualist scenarios rather than merely their non-actualization.

While Frank Jackson proposed a solution involving "minimal physical duplicates" (worlds containing nothing more than what is required by the physical facts), David Chalmers offered an alternative formulation based on "positive facts." Chalmers argues that supervenience theses should apply only to positive facts—facts that cannot be negated merely by enlarging a world (e.g., "Jones is in pain")—and not to negative facts (e.g., "There are no Cartesian souls").

Despite Chalmers's proposal, it faces three primary objections:

The Blocker Objection (Leuenberger): Chalmers's account is claimed to be inferior to Jackson's because it is not "blocker-friendly." It allegedly fails to classify a world as physicalist if mental facts are only "weakly necessitated" by physical facts (i.e., necessitated only in the absence of non-physical "blockers").
The Hawthorne Objection: Blockers pose a specific challenge to Chalmers's account regarding "Restricted Materialism" (R.M.), suggesting that Chalmers's formulation incorrectly validates materialism in worlds where blockers prevent certain mental properties from being instantiated.
The Incoherence Objection (Rabin): Chalmers's definition of positive facts is alleged to be internally inconsistent when combined with the possibility of blockers and the mereological relation of proper parthood between worlds.

Methodology and Argumentation
The author employs conceptual analysis and modal logic to evaluate competing formulations of physicalism. The paper proceeds by systematically reconstructing and refuting the objections against Chalmers's positive-fact formulation.

Section 2 (The Problem of Extras): The author formalizes the supervenience thesis and demonstrates how Chalmers's restriction to positive facts ( $F^+$ ) avoids the Problem of Extras. Unlike the standard formulation, Chalmers's account ( $P_{\alpha} \rightarrow S$ for all $S \in F^+_{\alpha}$ ) does not require the non-existence of Cartesian souls to be physically necessary, as this is a negative fact.
Section 3 (The Challenge of Blockers):
- Against Leuenberger: The author argues that the preference for Jackson's account rests on the controversial assumption that physicalism is "blocker-friendly" (i.e., that weak necessity is sufficient for physicalism). The author rejects this assumption based on three grounds:
  1. Causal Closure: If mental properties derive causal powers from physical properties plus the absence of blockers, this violates the causal closure of the physical. If they derive powers solely from physical properties, the presence of blockers becomes causally inert, contradicting their definition.
  2. Simplicity: A world where physical facts strongly necessitate all other facts (non-blocker-friendly) is more parsimonious than one requiring the additional condition of "no blockers."
  3. Identity Thesis: Physicalism often relies on the identity of mental and physical properties, which implies strong necessity. Blocker-friendly accounts exclude this central thesis.
- Against Hawthorne: The author contends that Hawthorne's objection relies on an overly demanding conception of Restricted Materialism (R.M.) that requires the physical explanation of uninstantiated properties. The author proposes a reformulation of R.M. where a property is materialistic if, whenever it is instantiated in any possible world, its instantiation is strongly necessitated by the physical facts of that world. Under this view, Chalmers's account correctly handles the scenario.
Section 4 (Further Challenges):
- Against Francescotti (Problem of Extras is not substantive): The author refutes the claim that physical facts must necessitate negative facts. The author argues that the truth of negative facts (e.g., "no ghosts exist") is supported by philosophical considerations (parsimony, causal criteria) rather than being entailed solely by physical premises.
- Against Rabin (Incoherence): The author addresses the alleged incoherence between positive facts, blockers, and proper parthood. The author rejects the premise that a world $w$ (where Jones is in pain) is a "proper part" of a world $w'$ (where a blocker prevents pain). Drawing on mereological principles regarding location and identity, the author argues that if the intrinsic features of $w$ (the pain) are altered in $w'$ , $w$ cannot be a proper part of $w'$ . Thus, the definition of positive facts remains consistent.

Key Contributions and Results

Defense of Chalmers's Formulation: The paper successfully defends Chalmers's positive-fact formulation against the "Problem of Extras," demonstrating that it avoids the counter-intuitive implication that the non-existence of dualist entities is physically necessary.
Rejection of Blocker-Friendliness: The paper provides a robust argument that physicalism should not be "blocker-friendly." It establishes that weak necessity is insufficient for physicalism, thereby neutralizing Leuenberger's claim that Jackson's minimal-world approach is superior.
Refinement of Restricted Materialism: The author offers a cross-world reformulation of Restricted Materialism that resolves Hawthorne's blocker-based objection, showing that Chalmers's account can accommodate worlds with blockers without validating non-physicalist scenarios.
Clarification of Mereology in Modal Contexts: The paper contributes to the understanding of "proper parthood" between possible worlds, arguing that it cannot be defined merely by the inclusion of positive propositions if the intrinsic states of the parts are altered by blockers.

Significance and Claims
The paper concludes that Chalmers's positive-fact formulation provides a "simple and defensible account of physicalism." It claims superiority over Jackson's formulation because it avoids the theoretical complexities of "minimality" or "that's-all" clauses and successfully withstands objections regarding blockers and internal coherence. The author asserts that formulating physicalism in terms of positive facts resolves the Problem of Extras without sacrificing the core physicalist commitment that the physical facts of the world determine all other facts (specifically, the positive facts) through strong necessity. The paper does not claim to solve the "base problem" (defining the physical) or the "domain problem" (scope of physicalism), but rather focuses strictly on the "relation problem" (the nature of the supervenience relation).

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