Take a moment to reflect on this significant quote from critical philosophy:

“All the objects of human reason or enquiry may naturally be divided into two kinds, to wit, relations of ideas, and matters of fact. Of the first kind are the sciences of Geometry, Algebra, and Arithmetic … [which are] discoverable by the mere operation of thought. … Matters of fact, which are the second object of human reason, are not ascertained in the same manner; nor is our evidence of their truth, however great, of a like nature with the foregoing. … [Therefore] if we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.” (David Hume, An Enquiry Concerning Human Understanding, section IV part I)

I have had a tumultuous relationship with this passage. When I first encountered it during my philosophical studies, it resonated deeply: YES! I thought, this is the truth! Down with metaphysics!

However, years later, I questioned whether Hume’s Fork should apply to his own Enquiry Concerning Human Understanding, which seems devoid of either relations of ideas (like mathematics and formal logic) or matters of fact (such as scientific evidence). Yet, this work remains a cornerstone of early modern philosophy!

Currently, I’m reassessing this stance. I believe that Hume’s Fork may not be universally applicable. It certainly doesn’t extend to literary criticism and may not apply to philosophy when it critiques various concepts—an approach I increasingly advocate for modern philosophy. (A notable exception exists for philosophy as a way of living, another significant branch.)

I have similarly grappled with a notion articulated by early 20th-century logical positivists: the verification principle. This principle posits that only statements verifiable through empirical means carry meaning, with logical truths as the sole exception. This principle is essentially a variant of Hume’s Fork.

The logical positivists themselves refined their principle to preempt many critiques, asserting that theology and metaphysics, along with aesthetics and ethics, are excluded. However, they did not argue that such statements are entirely devoid of meaning; rather, they do not convey truth values or factual information. This assessment seems accurate to me.

A common, albeit superficial, criticism of the verification principle is its self-exclusion. While true, it overlooks the core argument; logical positivism critiques epistemology rather than claiming to provide truths, whether mathematical or empirical.

Decades after the decline of logical positivism (and its close relative, logical empiricism), we can look to James Ladyman and Don Ross in Every Thing Must Go: Metaphysics Naturalized. In their concluding section titled “Neo-Positivism,” they assert:

“Taking naturalism seriously in metaphysics is equivalent to adopting a verificationist attitude towards both science and metaphysics.”

In essence, both science and metaphysics assert truth-like claims about the world, and our response should be to seek evidence.

Importantly, Ladyman and Ross's version of verificationism shifts focus from sentence meaning to epistemic value. A neo-positivist, or neo-Humean, does not dismiss metaphysical statements as meaningless; rather, they argue that such statements lack epistemic substance. This renders them impractical, suggesting that we should reconsider the entire metaphysical enterprise.

For example, consider the well-known concept of “philosophical zombies,” introduced by David Chalmers in discussions surrounding consciousness. This idea posits creatures, or p-zombies, that are physically indistinguishable from us yet exist in non-conscious states. Chalmers is not asserting their existence within our physical universe governed by its laws; he merely suggests that it is conceivable they might exist in some alternate universe.

From this seemingly modest premise, he launches a comprehensive critique against physicalism in the philosophy of mind, famously arguing that science cannot resolve what he terms the “hard” problem of consciousness, thereby leading us to accept that mental properties differ fundamentally from physical properties (known as property dualism).

This conclusion seems overly ambitious, drawing vast implications from a simple thought experiment grounded in conceivability. Despite the vague nature of the concept, nothing Chalmers proposes can be empirically verified, nor does it arise from mathematical or logical relations. Does this render it nonsense, as logical empiricists would claim? Not at all; I find Chalmers’s arguments comprehensible. Should we then follow Hume’s advice and discard his works? Absolutely not—I oppose any form of book burning.

However, Chalmers’s ideas should not be taken seriously, as he attempts to assert insights about reality without engaging with our sole means of accessing that reality: empirical evidence. His speculations are, in the apt words of physicist Wolfgang Pauli, “not even wrong.”

Why does this criticism not extend to Ladyman and Ross’s criterion of epistemic verifiability? They do not claim to make factual assertions about reality; instead, they engage in the philosophical equivalent of literary analysis, where demanding empirical evidence would be nonsensical.

For those intrigued by this perspective, I have previously discussed Ladyman, Ross, and Kincaid’s subsequent work, Scientific Metaphysics, in a book club series.

It's worth noting that it is not only Chalmers (or Philip Goff with his panpsychism) who faces challenges. Concepts often deemed scientific, such as string theory and parallel universes, similarly lack empirical contact, thus failing to meet the criteria for science.

What then becomes of metaphysics? Should we cease its practice altogether? Not quite. I propose that metaphysics has reached a point where it can be effectively "dissolved" into various natural sciences. Just as natural philosophy has evolved into distinct scientific fields like physics, chemistry, geology, and biology, starting with Galileo, we can now consider a similar transformation for metaphysics.

More specifically, I argue that metaphysics, as a broad discipline, can now integrate into science (in its broadest sense) and the associated field of philosophy of science. The philosophy of mind is increasingly merging with neuroscience, cognitive science, and possibly evolutionary biology. Ontology, which investigates existence, now falls under the domain of fundamental physics, among other areas.

By "dissolution," I mean that metaphysical questions can be categorized into those that Chalmers engages with—questions that are "not even wrong"—or those discussed by Ladyman and Ross, where answers arise from a critical (i.e., philosophical) examination of insights provided by physics, biology, or cognitive science.

Of course, some questions may be empirically investigable but remain unresolved, such as the origins of life and the universe. If these issues remain unanswered, it may be due to either our cognitive limitations or the loss of historical evidence that would have offered empirical insights.

What I have outlined may seem like a simplistic form of scientism, a notion that Ladyman and Ross unfortunately endorse. However, I do not share this view, as it is clear from my writings. Scientism posits that only scientifically answerable questions hold significance. I firmly disagree with this assertion.

This essay itself illustrates my point. Just as Hume’s Enquiry lacked relations of ideas or matters of fact, my writing does not rest on empirically verifiable statements. This is not problematic, as I engage in philosophical criticism rather than making definitive, truth-bearing claims about reality.

This means there remains substantial work for philosophers, as philosophy did not evaporate after the Scientific Revolution, despite the dissolution of natural philosophy into various sciences.

Moreover, every time a branch of philosophy evolves into a scientific discipline, it generates new philosophical inquiries, leading to the emergence of fields like the philosophy of biology, philosophy of physics, and philosophy of language. For further insight into how philosophy progresses, see here.

I hope my argument is not misconstrued as special pleading. Let me offer one more example: I believe ethics remains a valid area for philosophical exploration and is not in danger of dissolving into biology, sociology, or similar fields. Ethics fundamentally involves critical reasoning about actions in various situations, based on our values.

While ethical decision-making should certainly draw from empirical evidence, as it pertains to real-life contexts, one could argue that science provides empirical explanations for the origins (via evolutionary biology) and functions (through neuroscience) of ethical instincts and reasoning.

Nevertheless, ethical decision-making does not hinge on moral truths (I do not subscribe to ethical realism). If it did, we would pose the same question to ethicists as we do to metaphysicians: how do you know? Instead, ethics revolves around what logically follows from our values and the factual landscape. Therefore, ethics represents a type of philosophical discourse that does not conflict with the epistemic verificationism advocated by Ladyman and Ross.

In light of all this, we can refine Hume’s fork as follows:

If we examine any volume (or video or podcast) on metaphysics, we should ask: (i) Does it contain formal logical or mathematical reasoning? No. (ii) Does it include empirically based reasoning regarding matters of fact and existence? No. (iii) Does it confine itself to critical philosophy? No. We should disregard it (but refrain from burning it!), as it likely consists solely of sophistry and illusion.