Timeline of type B physicalism

This is a timeline of Type-B physicalism, the view in philosophy of mind holding that phenomenal consciousness is identical to physical states as a matter of a posteriori metaphysical necessity, even though this identity is not knowable a priori. The timeline traces the position from its mid-century origins in identity theory (Place, Smart, Lewis) through Chalmers' 1999 formalization of the Type-A/Type-B distinction, the phenomenal concept strategy, and ongoing debates over the explanatory gap.

Sample questions

The following are some interesting questions that can be answered by reading this timeline:

• What are the major developments in the history of Type-B physicalism?
  • Sort the full timeline by "Year" (its default order).
  • You will see identity theory and functionalism emerge from the 1950s through the 1980s, the Type-A/Type-B terminology get formalized in the 1990s, and the phenomenal concept strategy and its rivals develop from 2000 onward.
• What are the main thought experiments that Type-B physicalism has had to respond to?
  • Sort the full timeline by "Event type" and look for the group of rows with value "Thought experiment".
  • You will see the two thought experiments that pose the specifically modal and subjective-character challenges Type-B physicalism was developed to answer.
• What is the knowledge argument, and how does it relate to Type-B physicalism?
  • Sort the full timeline by "Event type" and look for the group of rows with value "Knowledge argument".
  • You will see the publication that introduces the Mary thought experiment, which later Type-B responses were developed to answer.
• When was the Type-A/Type-B/Type-C classification scheme introduced?
  • Sort the full timeline by "Event type" and look for the row with value "Classification introduced".
  • You will see the book that sets out the underlying distinction, with the explicit labels following a few years later in a reply to critics.
• What publications make up the ongoing debate between Type-B physicalists and their critics?
  • Sort the full timeline by "Event type" and look for the group of rows with value "Publication".
  • You will see the bulk of the timeline: the papers and books that carry on this debate across the decades.

Big picture

Time period	Development summary	More details
Before 1990	Identity theory and the challenges that would define Type-B	Mind-brain identity theory and early functionalism develop (Place 1956, Smart 1959, Lewis 1966–72, Armstrong 1968, Putnam 1967) largely without the specifically modal and epistemic pressure that later defines Type-B materialism. That pressure arrives at the end of this period: Kripke's modal argument against psychophysical identity (1980), Nagel's "what it's like" (1974), Jackson's knowledge argument (1982), and Levine's naming of the "explanatory gap" (1983) together force a reply that mere identity theory and functionalism had not needed to give.
1990–1999	The phenomenal concept strategy and the Type-A/Type-B labels	Loar (1990), Papineau (1993), and Hill and McLaughlin (1999) develop the phenomenal concept strategy — explaining away the apparent phenomenal/physical gap by appeal to the special nature of phenomenal concepts rather than to non-physical properties — as the dominant Type-B response. Chalmers' The Conscious Mind (1996) restates the case against this kind of reply, and his 1999 "Materialism and the Metaphysics of Modality," replying to a symposium of critics, formally introduces "Type-A materialism" and "Type-B materialism" as terms of art, fixing the vocabulary this timeline is organized around. Block and Stalnaker (1999) and Balog (1999) stake out the main lines of defense.
2000–2009	Consolidation and the first "master argument"	The phenomenal concept strategy is elaborated and defended at greater length (Papineau 2002, Stoljar 2005, Balog 2009) and begins appearing in handbooks, while Chalmers and Jackson (2001) press the demand for a priori conceptual analysis and Chalmers (2007) formulates a "master argument" meant to show that no version of the strategy can be both physically explicable and epistemically adequate — the dilemma that structures most subsequent debate. Rival or adjacent positions (Tye's representationalism, Kirk 2008, Levin 2008, Stoljar's "epistemic view" in Ignorance and Imagination, 2006) also take shape in this decade.
2010–2019	Responses to the master argument, and direct attacks on Type-B	Coleman (2009) and Balog (2012) answer Chalmers' 2007 dilemma on behalf of the constitutional account, while Howell (2013) and Tomasik (2014) develop or critique Type-B from new angles ("subjective physicalism," a Type-A "zombic hunch" argument). For the first time, Type-B itself — not just Type-A — comes under direct attack: Yetter-Chappell's "Dissolving Type-B Physicalism" (2017) argues there is no ideal epistemic gap for any phenomenal-concept theory to explain.
2020–present	New categories, and the question of whether the dispute is terminological	The original three-way Type-A/B/C taxonomy is joined by new proposed categories (Moorfoot's "Type-R physicalism," 2024) and by Stoljar's continued development of the "epistemic view" (2020) as something distinct from mainstream Type-B. Several recent papers (Botin 2023/2024, Zhou 2025, Brown and Papineau 2024) revisit Goff's "revelation challenge" and increasingly ask whether the long-running Type-B/illusionism dispute is substantive at all, or merely a disagreement about which concepts to keep.

Full timeline

Inclusion criteria

We include:

• Publications and other events that introduce, name, or formally define a position that this timeline tracks — identity theory, functionalism, Type-A/Type-B/Type-C materialism, and later refinements such as "Type-R physicalism" or the "epistemic view."
• Publications that develop or defend some version of the phenomenal concept strategy: accounts of phenomenal concepts (recognitional, quotational, constitutional, indexical, direct-reference, etc.) offered specifically to explain the epistemic/conceptual gap between phenomenal and physical descriptions in a way compatible with physicalism.
• The specific anti-physicalist arguments that Type-B materialism was developed to answer, and that it continues to be tested against: Kripke's modal argument, Nagel's "what it's like" argument, Jackson's knowledge argument, Levine's explanatory gap, and Chalmers' conceivability/zombie and "master" arguments.
• Direct responses to, critiques of, or defenses against the above — including papers that argue against Type-B materialism from a Type-A, Type-C, illusionist, or other anti-physicalist position, as long as they engage the specific phenomenal-concept/explanatory-gap machinery this timeline tracks, not consciousness or physicalism in general.
• Retrospective, survey, or handbook treatments that explicitly situate Type-B materialism relative to rival positions (Type-A, Type-C, Russellian monism, illusionism, panprotopsychism), even when the treatment itself doesn't advance a new argument.
• Precursor publications (identity theory, functionalism, multiple realizability) that later Type-B physicalists explicitly build on or that Chalmers and others cite as exemplars of the position, even though these publications predate the Type-A/Type-B terminology itself.

We do not include:

• General surveys, textbooks, or encyclopedia entries on consciousness, physicalism, or philosophy of mind that do not specifically engage the a priori/a posteriori entailment distinction, the phenomenal concept strategy, or the explanatory gap.
• Publications defending or attacking physicalism using an apparatus unrelated to phenomenal concepts or the explanatory gap — for example, disjunctive-property or grounding-based arguments (e.g. Baysan and Wildman on disjunctive physicalism) that don't engage this timeline's specific machinery, even when they defend a conclusion ("physicalism is true/false") that sounds adjacent.
• Neuroscientific or empirical work on the neural correlates of consciousness, except where a philosopher explicitly uses it to argue for or against Type-B materialism specifically (rather than for physicalism or consciousness science in general).
• General treatments of functionalism, multiple realizability, or identity theory that do not bear on the Type-A/Type-B distinction or the explanatory gap — most of 20th-century philosophy of mind is out of scope except where it's a direct precursor (see "We include," above).
• Events belonging more properly to a different, already-existing timeline on this wiki — e.g. material that belongs on Timeline of type A physicalism or Timeline of David Chalmers' work on consciousness because it concerns Type-A materialism or Chalmers' broader philosophical output without specific bearing on the type-A/B/C distinction itself.
• Popularizations, interviews, or media coverage that merely restate the Type-A/B/C classification for a general audience without adding or responding to an argument, unless (as with the 2022 Chalmers interview already on this timeline) the speaker is a principal figure in the debate directly updating or restating their own position.

Year	Event type	Details
1956	Publication	British philosopher U. T. Place publishes "Is Consciousness a Brain Process?" in the British Journal of Psychology, arguing against both behaviorism and dualism that consciousness should instead be identified with a pattern of brain activity — an influential statement of identity theory, also known as "Australian materialism." Distinguishing the "is" of definition from the "is" of composition, Place argues that "consciousness is a process in the brain" is neither self-contradictory nor self-evident but a reasonable scientific hypothesis, analogous to "lightning is a motion of electric charges."
1959	Publication	Australian philosopher J. J. C. Smart publishes "Sensations and Brain Processes" in the Philosophical Review (Vol. 68, No. 2), defending the mind-brain identity theory by arguing that sensations are identical with brain processes — explicitly framing the paper as an attempt to present Place's 1956 thesis "in a more nearly unobjectionable form" and to answer objections Place had not considered. Smart introduces the crucial notion of "topic-neutral" language: when someone says "I have a yellowish-orange after-image" they are saying something like "there is something going on which is like what goes on when I see an orange," using quasi-logical words that are neutral between dualist and materialist metaphysics, explaining how sensations can be brain processes without the sensation statement meaning the same as the brain-process statement. Smart's term for sensations viewed as lying outside the physicalist picture — "nomological danglers" (borrowed from Feigl) — became one of the most quoted phrases in the philosophy of mind literature.
1966	Publication	American philosopher David Lewis publishes "An Argument for the Identity Theory" in the Journal of Philosophy, arguing — more strongly than Place or Smart — that anyone who accepts the materialistic working hypothesis that all physical phenomena have purely physical explanations is not merely *free* to accept identity theory but is *compelled* to: the definitive characteristic of any experience is its causal role, and if materialists believe those causal roles belong to physical states, then experiences simply are those physical states. The causal-role definition of experience introduced here becomes the direct foundation of Lewis's later analytic functionalism in "Psychophysical and Theoretical Identifications" (1972, below).
1967	Publication	American philosopher Hilary Putnam publishes "Psychological Predicates" in W. H. Capitan and D. D. Merrill's edited volume Art, Mind, and Religion (later reprinted as "The Nature of Mental States" in Putnam's Mind, Language, and Reality), introducing the multiple realizability argument against type-identity theory: pain cannot be identical to any single physical-chemical brain state because the same psychological predicate applies to organisms with radically different physiologies — a mammal and an octopus can both feel pain despite having no shared brain-state type, since (Putnam notes) their eyes evolved convergently from entirely different cell types despite performing the same function. Putnam proposes instead that pain is a "functional state of the whole organism," defined by its causal role within a system's Turing Machine-style functional organization rather than by its physical realization, and argues this hypothesis is superior to both the brain-state theory and behaviorist "behavior disposition" accounts (the latter cannot specify the relevant disposition except by circularly invoking pain itself). Putnam also directly rebuts J. J. C. Smart's objection that non-physical states are unintelligible, noting that functional states are neither mental nor physical per se. This multiple realizability argument would become the central challenge motivating the shift from strict type-identity theory toward the causal-role and functionalist analyses developed by David Armstrong (1968, above) and David Lewis (1970–72, below).
1968	Publication	Australian philosopher David Armstrong publishes A Materialist Theory of the Mind (Routledge & Kegan Paul), the most comprehensive book-length statement to date of "central-state materialism": working systematically through perception, belief, the will, bodily sensations, introspection, and the secondary qualities, Armstrong argues that mental states are contingently identical with states of the central nervous system, picked out by their causal role in producing behavior rather than by any distinctively mental essence — a materialism explicit about being compatible in principle with multiple physical realizations of the same mental state type, going beyond the narrower topic-neutral formulations of Place (1956, above) and Smart (1959, above). David Chalmers would later cite Armstrong, alongside David Lewis, as an exemplar of the position he formally labels Type-B materialism in The Conscious Mind (1996, below). A revised second edition, with a new preface in which Armstrong reflects on the book's impact and a bibliography of subsequent materialist-debate literature, was published by Routledge in 2002.
1970	Publication	American philosopher David Lewis publishes "How to Define Theoretical Terms" in the Journal of Philosophy (Vol. 67, No. 13), providing the formal apparatus later applied to psychophysical identifications — arguing that theoretical terms introduced by a new theory T are definable as the occupants of the unique causal roles specified by T, and that when a reducing theory shows what occupies those roles, the bridge laws connecting mental and physical states follow as deductive consequences rather than voluntary posits made for parsimony. This is the logical machinery that makes Lewis's 1972 "Psychophysical and Theoretical Identifications" (below) possible: the 1970 paper introduces the general method, the 1972 paper applies it specifically to folk psychology and neural states.
1972	Publication	David Lewis publishes "Psychophysical and Theoretical Identifications", arguing that psychophysical identities should be understood as implied by the theories that make them possible rather than posited independently, refining the Ramsey-style functional-definition approach from his 1966 and 1970 papers into the form most often cited by later Type-B physicalists.
1974	Thought experiment	Thomas Nagel publishes "What Is It Like to Be a Bat?", arguing that subjective experience presents a special explanatory challenge for physicalism — any reductive account of mind seems to leave out the subjective "what it is like" character of experience — and helping motivate later Type-B responses that try to preserve physicalism without denying this subjective character.
1980	Thought experiment	Saul Kripke publishes Naming and Necessity as a standalone book (Harvard University Press; the text of three 1970 Princeton lectures, first published in 1972, with only a new preface added here). Kripke argues that names and natural-kind terms are rigid designators, picking out the same referent in every possible world, with reference typically fixed by an initial baptism and passed on through a causal-historical chain rather than by a synonymous description — grounding the category of the necessary a posteriori: identities like "Hesperus is Phosphorus" or "heat is molecular motion" are necessary if true, though only knowable empirically. He then turns this against mind-brain identity theory: unlike heat, where the felt sensation is a contingent intermediary that could in principle have come apart from molecular motion, there is no such intermediary between a pain and the sensation of pain, so the usual move that rescues ordinary scientific identities cannot rescue "pain = C-fiber firing" — posing a specifically modal challenge that Type-B physicalists, not just dualists, have had to answer ever since.
1980	Publication	David Lewis publishes "Mad Pain and Martian Pain" in Ned Block's edited Readings in Philosophy of Psychology, Volume I, defending functionalism with two thought experiments: "mad pain," whose causes and effects are entirely atypical yet still counts as pain, and "Martian pain," realized in hydraulics rather than neurons. Lewis handles mad pain by making the causal-role definition population-relative (pain is whatever occupies the pain-role for one's species, even when this instance deviates from it) while Martian pain is secured by multiple realizability (1967, above); he insists throughout that to be in pain just is to feel pain, so a functionalist account of the causal role is automatically an account of how it feels. A 1983 postscript, added when the paper was reprinted in Lewis's Philosophical Papers, Volume I (Oxford University Press), answers Jackson's 1982 knowledge argument directly: Mary gains something on leaving the room, but, following Laurence Nemirow, what she gains is abilities (to recognize, imagine, predict), not propositional information physics left out — an early statement of the "ability hypothesis."
1982	Knowledge argument	Australian philosopher Frank Jackson publishes "Epiphenomenal Qualia" in The Philosophical Quarterly, introducing the Mary thought experiment: a scientist who learns every physical fact about colour vision from inside a black-and-white room yet appears to learn something new on finally seeing red, showing physicalism leaves something out. Jackson distinguishes this Knowledge argument from the related Modal argument and Nagel's 1974 "what is it like to be" argument (above), and defends epiphenomenalism about qualia against three standard objections, conceding it remains a mystery how such causally inert qualia fit into the scientific picture. Jackson's later reversal, and the Type-B responses to it (Lewis 1994, Papineau 2002, below), would make the Mary case the most influential knowledge-argument formulation in the subsequent literature.
1983	Publication	Joseph Levine publishes "Materialism and Qualia: The Explanatory Gap" in Pacific Philosophical Quarterly, coining the term "explanatory gap" while recasting Kripke's 1980 modal argument (above) as an epistemological rather than metaphysical problem: identities like "heat is molecular motion" are fully explanatory (nothing is left unaccounted for once we know the mechanism), whereas "pain is C-fiber firing" leaves the qualitative feel of pain unexplained by its physical or functional role, which Levine argues is what generates the felt contingency Kripke exploits. Levine contends this gap survives the move to functionalism (citing Ned Block's "Troubles with Functionalism") and, even if some psychophysical identity is true, makes it epistemically undeterminable which one is, concluding that only an eliminativist retreat from qualia could fully dissolve the problem for materialism — a weaker, purely epistemic conclusion than Kripke's, but one Levine frames as better capturing philosophers' persistent unease with physicalism, and the central challenge later Type-B physicalists (Loar, Papineau, Balog, and others, below) would need to answer directly.
1989	Publication	Colin McGinn publishes "Can We Solve the Mind–Body Problem?" in Mind, arguing that some natural, non-miraculous property P of the brain must explain consciousness, but that human minds are "cognitively closed" with respect to P: introspection cannot reveal it (phenomenology gives no access to the brain side of the link), and perception of the brain cannot reveal it either, since the senses disclose only spatial properties, which are the wrong category to render consciousness intelligible, and inference to the best explanation of purely physical data never needs to posit consciousness to explain it. McGinn argues this closure is principled rather than a temporary gap awaiting future neuroscience (deeper investigation of the brain has only made the mystery worse, not better), yet insists the resulting mystery is merely epistemic: our vacillation between finding the mind-brain link contingent (Descartes, Kripke) and suspecting some necessary supervenience reflects only our inability to grasp P, not an actual indeterminacy in nature. A naturalistic but non-Type-B response to the explanatory gap — closer to what Chalmers would later classify as Type-C materialism — that nonetheless sharpened the explanatory-gap debate Type-B physicalists needed to answer.
1990	Publication	Brian Loar publishes "Phenomenal States" in Philosophical Perspectives, arguing that anti-physicalist arguments rest on a "semantic minor premise" — that concepts connected only a posteriori must introduce distinct properties — which he rejects: phenomenal concepts are recognitional/imaginative concepts, cognitively independent of physical-functional concepts yet capable of directly designating the very same physical-functional properties those concepts pick out, with no need for a further non-physical reference-fixer. This lets Loar concede the "fundamental anti-physicalist intuition" (that physical knowledge never yields phenomenal knowledge a priori) while denying it shows phenomenal properties are non-physical, since the felt independence is a fact about our concepts rather than about the properties themselves — an influential early statement of the phenomenal concept strategy, developed partly against Kripke's 1980 argument (above) and anticipating later constitutional accounts such as Balog's (1999, 2012, below).
1993	Publication	David Papineau publishes "Physicalism, Consciousness, and the Antipathetic Fallacy", introducing the "antipathetic fallacy" — the idea that phenomenal and physical concepts of the same state feel intuitively distinct even though they refer to the same thing, producing an illusion of an explanatory gap — the core idea underlying all of Papineau's later Type-B defenses.
1993	Publication	David Papineau publishes Philosophical Naturalism, defending the view that consciousness, Meaning (philosophy of language), and knowledge can all be accommodated within a naturalist, physicalist worldview, and diagnosing the intuition that consciousness is non-physical as a confusion arising from the special structure of human imagination — the book-length companion to his AJP paper on the antipathetic fallacy published the same year.
1993	Publication	David Chalmers completes his PhD dissertation Toward a Theory of Consciousness at Indiana University, developing the supervenience-based framework and conceivability arguments that consciousness is not logically supervenient on the physical — the direct precursor to The Conscious Mind and the source of the conceivability arguments that would become the principal target of later Type-B responses.
1994	Publication	David Lewis publishes "Reduction of Mind", defending psychophysical reduction while emphasizing the distinction between conceptual and metaphysical necessity — Lewis explicitly addresses Frank Jackson's knowledge argument here, holding that Mary gains new abilities upon seeing color rather than new factual information, a key move in the Type-B response to the knowledge argument.
1995	Publication	David Chalmers publishes "Facing Up to the Problem of Consciousness" in the Journal of Consciousness Studies, coining the term "hard problem" to distinguish it from the "easy problems" (reportability, attention, access) that standard cognitive-scientific methods can explain by specifying a mechanism for the function. Any such functional explanation, Chalmers argues, leaves open why the performance of these functions is accompanied by experience at all — an explanatory gap (Levine's 1983 term, above) no physical account can close, since it remains conceivable that the same process occur without experience. Rather than deeming the problem intractable, Chalmers proposes treating experience as fundamental, governed by new "psychophysical" bridging laws, and sketches candidate principles (structural coherence, organizational invariance, a double-aspect theory of information) for what he calls "naturalistic dualism"; a footnote briefly dismisses Kripke-style a posteriori necessity as used by Hill, Levine, and Loar (above), a critique developed at length the following year in The Conscious Mind. The paper gives the hard-problem framing its public debut a year ahead of the book that would formalize the Type-A/Type-B distinction it structures.
1996	Classification introduced	David Chalmers publishes The Conscious Mind: In Search of a Fundamental Theory (Oxford University Press), arguing for property dualism via the conceivability-possibility argument: since it is conceivable that a world physically identical to ours lacks consciousness (a "zombie world"), and conceivability of this kind entails metaphysical possibility, facts about consciousness are further facts over and above the physical facts. In a footnote addressing physicalist responses that grant the conceivability of zombies but deny it shows anything, since psychophysical identities could be true as a posteriori necessities in the manner of Kripke's heat/molecular-motion case, Chalmers sets out the a priori/a posteriori entailment distinction this timeline is built around: such a view, he says, "must be combined with an appeal to strong metaphysical necessity in supporting the link between biochemistry and consciousness, inheriting all the problems with that view," and he names David Lewis and David Armstrong (1968, 1972, above) as exemplars of philosophers committed to it. Chalmers does not yet use the labels "Type-A" and "Type-B materialism" here; he introduces that explicit terminology only in his 1999 reply to critics (below), prompted directly by the responses of Hill and McLaughlin (1999, below) and others to this book.
1997	Publication	Christopher Hill publishes "Imaginability, Conceivability, Possibility and the Mind-Body Problem" in Philosophical Studies, answering Kripke's modal argument against type-materialism with two explanations of its apparent force: first, developing a footnote of Nagel's, that intuitions of disembodiment arise from splicing together a sympathetic image of a mental state with a perceptual image of the absence of its brain state, a general "image-splicing" mechanism independently known to produce false separability intuitions (e.g. heat from molecular motion); second, that the faculty of conception produces the same intuitions because sensory concepts like pain and neuroscientific concepts like C-fiber stimulation serve different classificatory roles and so lack any a priori tie, which on its own does not show the concepts fail to corefer.
1997	Publication	David Chalmers publishes "Moving Forward on the Problem of Consciousness", a lengthy response to 26 commentaries on his 1995 "Facing Up to the Problem of Consciousness", addressing deflationary critiques, non-reductive critiques, and positive proposals, sharpening the target that later phenomenal concept strategies would need to engage.
1998	Publication	Christopher Hill publishes "Chalmers on the Apriority of Modal Knowledge" in Analysis, examining Chalmers' 1996 argument for property dualism (above), which turns on the claim that we have a priori access to logical possibility: Chalmers holds that the facts of consciousness are reducible to physical/functional facts only if it is logically impossible for a world to match the actual world in all physical and functional respects while lacking consciousness, that this logical (im)possibility is testable a priori by conceivability, and that since we can conceive of physical/functional duplicates of ourselves lacking consciousness (zombies), such a world is logically possible — so consciousness does not reduce to the physical/functional. Hill focuses on two passages where Chalmers attempts to justify this a priori access to modal knowledge, and subjects them to critical scrutiny, even while noting that Chalmers himself does not regard these particular passages as his strongest case for the view — a direct challenge to the modal-epistemological foundations of the zombie conceivability argument that provoked Chalmers, in his reply the following year, to formalize the Type-A/Type-B materialism distinction explicitly.
1999	Publication	Christopher Hill and Brian McLaughlin publish "There Are Fewer Things in Reality Than Are Dreamt of in Chalmers's Philosophy" in a four-paper Philosophy and Phenomenological Research symposium on The Conscious Mind, rejecting Chalmers' conceivability-possibility argument by denying that conceiving a physical duplicate of our world lacking consciousness entails its logical possibility. Drawing on Nagel's 1974 distinction between sympathetic and perceptual imagination (above), they argue that imagining pain without its brain state requires jointly exercising two faculties whose separate reliability doesn't guarantee reliability in combination, and that sensory concepts (self-presenting, recognitional) and physical/theoretical concepts (always accessed indirectly) are governed by such different epistemic constraints that their joint conceivability creates no presumption of genuine possibility — so psychophysical identities can be true yet only knowable a posteriori, without the "strong metaphysical necessity" Chalmers attributes to such a view. They also defend Loar's 1990 phenomenal concept strategy (above) against Chalmers' direct objection to it, arguing that two concepts whose coreference can't be known a priori need not be associated with distinct reference-fixing properties, since the difference can instead lie purely in how each concept's reference was psychologically fixed. This symposium exchange directly provoked Chalmers' reply the same year formalizing the Type-A/Type-B materialism distinction (below).
1999	Publication	David Chalmers publishes "Materialism and the Metaphysics of Modality" in Philosophy and Phenomenological Research, replying to a symposium on The Conscious Mind (papers by Hill and Brian McLaughlin, Brian Loar, Sydney Shoemaker, and Stephen Yablo). Chalmers here explicitly names and defines the labels this timeline uses: type-A materialists deny that zombie and inverted-qualia worlds are coherently conceivable and deny that Mary gains any factual knowledge on seeing red; type-B materialists accept the conceivability of such worlds but deny their metaphysical possibility, holding that Mary gains only old knowledge in a new form. Most of the paper argues against the "strong necessities" that Hill/McLaughlin and Loar propose to ground type-B materialism, on the grounds that no a posteriori necessity escaping Chalmers' two-dimensional semantic framework has ever been independently motivated.
1999	Publication	Ned Block and Robert Stalnaker publish "Conceptual Analysis, Dualism, and the Explanatory Gap" in The Philosophical Review, rejecting what they call the "conceptual analysis thesis" shared by Levine, Jackson, and Chalmers: that closing the explanatory gap requires an a priori conceptual analysis of consciousness in physical terms. Drawing an analogy to how the explanatory gap for life was closed without any such analysis of "life," they argue reductive explanation does not require a priori bridge principles, so the unanalyzability of phenomenal concepts does not establish dualism.
1999	Publication	Katalin Balog publishes "Conceivability, Possibility, and the Mind-Body Problem", arguing that Chalmers' zombie conceivability argument is self-undermining because an analogous argument, run "in the mouth of a zombie," would lead to a false conclusion — an influential internal defense of physicalism against conceivability arguments that established Balog as a leading phenomenal-concept-strategy theorist.
2000	Publication	Michael Tye publishes Consciousness, Color, and Content (MIT Press), devoting Part I (chapters titled "Knowing What It Is Like" and "The Explanatory Gap as a Cognitive Illusion") to arguing that Jackson's knowledge argument and Levine's explanatory gap are intimately related problems best dissolved by treating the explanatory gap itself as a kind of cognitive illusion, rather than evidence of either a metaphysical gap (as Type-B physicalists like Levine hold) or no gap at all. Part II develops Tye's 1995 representationalism (phenomenal character as a kind of nonconceptual representational content) at length in response to objections, while Part III extends the view to an objectivist account of color and to the question of how far down the phylogenetic scale phenomenal consciousness extends. The "cognitive illusion" framing sits ambiguously between Type-A and Type-B materialism depending on whether illusion is read as denying any genuine epistemic gap (Type-A) or as explaining, without eliminating, a real but non-ontological one (Type-B).
2001	Publication	David Chalmers and Frank Jackson publish "Conceptual Analysis and Reductive Explanation", arguing that reductive explanation of the phenomenal requires a priori entailment from microphysical truths to phenomenal truths — directly rebutting Ned Block and Robert Stalnaker's 1999 Type-B defense and prompting extensive Type-B physicalist responses on the role of conceptual analysis in resisting the conceivability and explanatory-gap arguments.
2002	Publication	David Papineau publishes Thinking About Consciousness (Oxford University Press), opening with a causal argument for materialism: conscious states have physical effects; all physical effects have a complete physical causal history; and these effects aren't overdetermined by a second, distinct sufficient cause — so materialism follows, rejectable only at the cost of empirically implausible alternatives. Most of the book instead explains why dualism feels intuitively compelling despite being false, via "conceptual dualism" (distinct from property or ontological dualism): phenomenal and physical concepts pick out the same properties in two different ways, and phenomenal concepts are distinctive in incorporating a "faint copy" of the experience itself, generating the "intuition of distinctness" that makes dualism seem natural — extending the antipathetic-fallacy diagnosis of his 1993 paper (above) to book length. Reviewing it for the Times Higher Education Supplement, Susan Blackmore called it written more for philosophers than scientists, and while granting it explained why the explanatory gap feels persuasive, doubted Papineau's view of consciousness as an inherently vague concept would discourage neuroscientific efforts to pin it down.
2005	Publication	Daniel Stoljar publishes "Physicalism and Phenomenal Concepts" in Mind & Language, dividing what he calls the phenomenal concept strategy into several versions and arguing that none succeeds in defending physicalism against anti-physicalist arguments, since phenomenal concepts cannot do the explanatory work Type-B physicalists (Loar, Papineau, and others, above) require of them. Stoljar closes by contrasting the strategy with a rival he calls the "missing concept strategy" — on which our inability to deduce phenomenal truths from physical ones reflects ignorance of further, currently unconceptualized physical or topic-neutral truths rather than anything special about phenomenal concepts themselves — and argues this alternative gives a more plausible response to the explanatory gap, the view he develops at book length the following year in Ignorance and Imagination (2006, below).
2006	Publication	Torin Alter publishes "Does Representationalism Undermine the Knowledge Argument?" in the same volume, responding to Frank Jackson's own reversal on the argument he originally formulated: Jackson now rejects the knowledge argument, arguing it rests on a mistaken conception of sensory experience that should be replaced by representationalism (phenomenal states are simply representational states), and that mental representation is physically explicable. Alter examines whether this representationalist route actually succeeds in undermining the argument without covertly relying on the same phenomenal-concept resources Loar, Papineau, and others (above) already use, or whether representationalism alone is insufficient to close the epistemic gap Jackson's Mary case opens.
2006	Publication	Daniel Stoljar publishes Ignorance and Imagination: The Epistemic Origin of the Problem of Consciousness (Oxford University Press), arguing that the mind-body problem arises not because experience is non-physical, nor because physicalism requires rejecting the reality of experience or revising how we talk about consciousness, but because we are ignorant of certain relevant physical facts; on this "epistemic view," closing that ignorance gap, rather than positing new ontology, would dissolve the problem.
2007	Publication	David Papineau publishes "Phenomenal and Perceptual Concepts" in Torin Alter and Sven Walter's edited volume Phenomenal Concepts and Phenomenal Knowledge, extending the comparison between phenomenal and perceptual concepts and directly responding to David Chalmers' argument against the Type-B phenomenal concept strategy.
2007	Publication	David Chalmers publishes "Phenomenal Concepts and the Explanatory Gap" in Torin Alter and Sven Walter's edited Phenomenal Concepts and Phenomenal Knowledge, presenting a "master argument" (building on his 1999 critique of Loar, above) intended to show that no version of the phenomenal concept strategy can simultaneously explain our epistemic situation regarding consciousness and be physically explicable itself: for any candidate thesis C attributing special features to phenomenal concepts, either it is conceivable that a physical duplicate lacks C (so C is no more physically explicable than consciousness itself), or it is not conceivable (in which case a "zombie" would possess C yet manifestly fail to share our epistemic situation, since Zombie Mary gains no significant new introspective knowledge on leaving her room). Applying this dilemma to specific proposals on the timeline, Chalmers argues Loar's recognitional-concept account (1990, above) and Hill's dual-conceptual-role account (1997, above) are each either "thin" (physically explicable but epistemically impotent) or "thick" (epistemically adequate but no more physically tractable than phenomenal consciousness), and that Papineau's quotational account (2002, above) survives only as a thin, bottom-up construction that cannot explain Mary's distinctive knowledge — directly targeting the Type-B phenomenal-concept defenses collected in the same volume.
2008	Publication	British philosopher Robert Kirk publishes "Physicalism and Phenomenal Consciousness" in Teorema (Vol. 27, No. 3, a special issue on "La conciencia fenoménica y el naturalismo contemporáneo" / "Phenomenal Consciousness and Contemporary Naturalism"), addressing the influential anti-physicalist argument from transposed and absent qualia: (1) if physicalism is true, transposed and absent qualia are impossible; (2) transposed and absent qualia are possible; (3) therefore physicalism is false. Kirk argues that physicalism commits its defenders to a "strict implication thesis" — the physical facts strictly imply all the mental facts — under a strong reading of premise (1), and that neither classical identity theory nor a posteriori (Type-B) physicalism escapes this commitment, contrary to what their proponents often suppose; defeating the argument therefore requires directly disproving premise (2), a task Kirk does not attempt in this paper. He further argues, however, that strict implication does not itself require analytic (a priori) links between the physical and the mental, so those who demand full "transparency" between physical and phenomenal concepts are imposing an unnecessary and overly strong requirement on physicalists.
2008	Publication	Janet Levin publishes "Taking Type-B Materialism Seriously" in Mind & Language (Vol. 23, No. 4, pp. 402–425), defending Type-B materialism against anti-physicalist objections (grounded in the conceivability of zombies, Mary's ignorance, and the explanatory gap) by arguing that these objections share a common flaw: they fail to take the central, distinctive features of Type-B materialism — namely its claim that phenomenal concepts have no a priori connections to physical or functional concepts, despite phenomenal states being necessarily identical to physical states — sufficiently seriously. Levin closes by conceding that Type-B materialism raises further questions of its own, and suggests that what stands in the way of a fully adequate naturalistic account of phenomenal states may be exactly this propensity among philosophers to not take Type-B materialism as seriously as it deserves — a preview of the argument she would return to a decade later in her 2018 "Once More Unto the Breach" (above/below).
2009	Publication	Katalin Balog publishes "Phenomenal Concepts" in The Oxford Handbook of Philosophy of Mind, surveying rival accounts of phenomenal concepts (direct-reference accounts from Tye, Perry, and Aydede/Güzeldere; special-mode-of-presentation accounts from Peter Carruthers (philosopher)) against a checklist of features any theory must explain — acquaintance, asymmetric epistemology, infallibility, the conceivability of zombies, and the explanatory gap — before developing her own "constitutional account," on which phenomenal concept tokens are partly constituted by the very experiences they refer to, akin to how a quotation is partly constituted by the expression quoted. Balog argues this constitutional structure (developed independently by Hill and McLaughlin, 1999, above, and by Papineau, 2002, above) explains the infallibility and semantic stability of phenomenal judgments and underwrites a Type-B response to the conceivability argument, while closing with Levine's (2001, 2007) objection that constitution alone cannot explain the "substantivity" of acquaintance, and Chalmers' (2007, above) master-argument dilemma against the entire phenomenal concept strategy.
2009	Publication	Sam Coleman (Birkbeck College, University of London) publishes "Chalmers's Master Argument and Type Bb Physicalism," responding directly to David Chalmers' 2007 dilemmatic "master argument" against every version of the phenomenal concept strategy (above). Chalmers' argument forces phenomenal concept strategists to choose between two unpalatable "horns": either treat phenomenal concepts as themselves physically explicable (horn a), which Chalmers argues undermines their ability to explain the epistemic situation regarding consciousness, or deny they are physically explicable (horn b), which undermines physicalism itself. Coleman explores and defends a position he dubs "Type Bb" — for Type-B physicalists willing to embrace horn b of the dilemma — and argues that Chalmers fails to demonstrate any genuine incoherence in the position of a Type-B physicalist who takes this route while still relying on the phenomenal concept strategy.
2010	Publication	Daniel Stoljar publishes Physicalism (Routledge, "New Problems of Philosophy" series), a systematic introduction built around an argument that no formulation of physicalism is both true and deserving of the name: starting-point physicalism (physical properties as those of ordinary objects) is refuted by physics itself, while any liberalized "theory view" (physical properties as whatever a true or possible physical theory posits) ends up either true in worlds where physicalism should be false or false in worlds where it should be true. Stoljar argues this undermines the standard picture of physicalism as a scientifically-grounded worldview generating "placement problems" for consciousness, meaning, and morality, and defends his own diagnosis of the hard problem of consciousness (developed independently in his 2006 Ignorance and Imagination) as resting on ignorance of relevant physical facts rather than a genuine ontological gap — an epistemic view he distinguishes from, while situating among, mainstream Type-B phenomenal-concept strategies. Against the conceivability argument he argues that physicalism typically plays only an inessential role in such arguments, which really turn on whether consciousness is fundamental rather than on whether it is specifically physical.
2011	Publication	David Papineau publishes "Phenomenal Concepts and the Private Language Argument" in American Philosophical Quarterly, first motivating phenomenal concepts via a variant of Jackson's knowledge argument: unlike Mary, "Marianna" is shown a coloured paper without being told its colour, coins a new concept Φ for the resulting experience, and only later learns Φ is caused by red things — a genuine piece of new knowledge, but (Papineau argues) only at the level of concepts, not reference, since the underlying material property she comes to call Φ is one she already knew of under a physical description, blocking Jackson's anti-materialist conclusion. Papineau then argues, against his own camp, that positing such concepts is inconsistent with Wittgenstein's private language argument, concluding by asking whether this tells against phenomenal concepts or against Wittgenstein — a self-critical stress-test of the Type-B framework from inside it.
2012	Publication	Katalin Balog publishes "In Defense of the Phenomenal Concept Strategy" in Philosophy and Phenomenological Research, responding to David Chalmers' 2007 master argument on behalf of the "constitutional" account of phenomenal concepts (Hill and McLaughlin 1999; Block 2007; Papineau 2002, 2007), on which a token phenomenal concept is partly constituted by the very experience it refers to. Balog concedes Chalmers' dilemma, that a phenomenally-conceptualized CPhen is not physically explicable while a physically-conceptualized CPhys cannot explain our epistemic situation, but argues this new explanatory gap between CPhen and the physical facts poses no greater threat to physicalism than the original gap between phenomenal and physical facts, since neither side can non-question-beggingly refute the other's core assumption.
2012	Publication	Janet Levin (University of Southern California) publishes "Do Conceivability Arguments against Physicalism Beg the Question?" in Philosophical Topics (Vol. 40, No. 2, "Consciousness" special issue, pp. 71–89), examining an increasingly popular Type-B response to conceivability arguments (such as Chalmers' zombie argument and Kripke's modal argument): the claim that these arguments' conceivability premises beg the question, since one's ability to conceive of zombies or disembodied minds depends entirely on one's antecedent theoretical commitments about the nature of consciousness, and so cannot legitimately be used to draw conclusions about whether conscious experiences could be physical. Rather than simply endorsing this popular "question-begging" response, Levin raises critical questions about various versions of it and argues that physicalists have more promising ways available to them for disarming conceivability arguments against physicalism.
2012 (January 15)	Publication	Adrian Boutel (Christ's College, University of Cambridge) publishes "How to be a type-C physicalist" in Philosophical Studies (Vol. 164, No. 2, pp. 301–320), defending a version of physicalism that reconciles the "a priori entailment thesis" (APET) — the claim that the complete truths of physics imply every other truth a priori, such that a demon with only complete physical knowledge could deduce every truth about the world ("cosmic hermeneutics") — with "analytic independence," the popular Type-B physicalist explanation for the apparent epistemic gap between phenomenal and physical truths, which is generally taken to be incompatible with APET since the demon's deductions seem to presuppose analytic connections between physical and phenomenal terms that Type-B physicalists deny exist. Boutel argues that implications from the complete truths of physics to phenomenal truths cannot be a posteriori in the Kripke style, since (by the physicalist's own lights) the complete truths of physics already fix any relevant a posteriori facts about the reference of phenomenal terms; he then shows how the physicalist can exploit this fixing of reference to deduce that phenomenal and physical terms co-refer, opening the way for a "Type-C" physicalism that accepts in-principle a priori deducibility while still appealing to Type-B-style analytic independence to explain why non-demons like us find phenomenal-physical connections impossible to see a priori in practice.
2013	Publication	Robert J. Howell publishes Consciousness and the Limits of Objectivity: The Case for Subjective Physicalism (Oxford University Press, 190pp.), defending a version of Type-B physicalism he calls "subjective physicalism": consciousness is wholly physical, but cannot be fully captured by any objective, third-person theory, because it can only be known "from the inside" via acquaintance. Howell defines physicalism using "Trans-World Disposition Sets" — physical properties are those whose causal/dispositional profile is exhaustively characterizable in terms of spatial and temporal implications — and argues that phenomenal properties, while physical, have an irreducibly subjective "aspect" that keeps a complete, objective physical theory from being what Howell calls "complete" in his technical sense (able to rule out all epistemically possible scenarios, such as zombies or spectrum inversion). Howell frames responses to Jackson's knowledge argument as splitting into "hardliners" (who deny Mary learns anything new, i.e. Type-A physicalists) and "epistemicists" (who grant she learns something epistemically significant but deny any metaphysical gap, i.e. Type-B physicalists), and situates his own view firmly in the latter camp while explicitly separating epistemic from metaphysical considerations. He argues subjective physicalism is preferable to eliminativism as the consequence physicalists should draw from taking consciousness seriously, while resisting the further step (which he associates with Russellian monism) of treating microphenomenal properties as fundamental and causally basic.
2014 (September 6)	Publication	Brian Tomasik publishes "Is There A Hard Problem of Consciousness?" on his website reducing-suffering.org (last substantively updated 21 June 2018), a long essay defending Type-A physicalism against Type-B by arguing that the "zombic hunch" — the intuition that phenomenal consciousness could be absent from a physical duplicate — is best explained as one more case of human intuition failing in a predictable, patterned way, analogous to optical illusions or pre-relativistic intuitions about absolute time. Tomasik argues Type-B physicalism amounts to disguised property dualism, posing a dilemma: if qualia supervene purely on functional organization, physical systems admit multiple equally valid functional interpretations with no fact of the matter about which is "the" conscious one; but if qualia depend on some further non-functional element (as in Ned Block's "biological" view), that element does no causal work and renders qualia epiphenomenal. He proposes an alternative "acquaintance knowledge" framework, following William James and Earl Conee, on which phenomenal experience is a first-person mode of being identical to being the physical system in question rather than third-personally derivable propositional knowledge, dissolving rather than bridging the explanatory gap. The piece also engages Sam Coleman's "Type Bb" response (above) to Chalmers' master argument, noting that Type-A and Type-B remain mutually self-consistent once one has settled whether to accept the "zombic hunch."
1993	Publication	David Papineau publishes Philosophical Naturalism, defending the view that consciousness, Meaning (philosophy of language), and knowledge can all be accommodated within a naturalist, physicalist worldview, and diagnosing the intuition that consciousness is non-physical as a confusion arising from the special structure of human imagination — the book-length companion to his AJP paper on the antipathetic fallacy published the same year.
1993	Publication	David Chalmers completes his PhD dissertation Toward a Theory of Consciousness at Indiana University, developing the supervenience-based framework and conceivability arguments that consciousness is not logically supervenient on the physical — the direct precursor to The Conscious Mind and the source of the conceivability arguments that would become the principal target of later Type-B responses.
1994	Publication	David Lewis publishes "Reduction of Mind", defending psychophysical reduction while emphasizing the distinction between conceptual and metaphysical necessity — Lewis explicitly addresses Frank Jackson's knowledge argument here, holding that Mary gains new abilities upon seeing color rather than new factual information, a key move in the Type-B response to the knowledge argument.
1995	Publication	David Chalmers publishes "Facing Up to the Problem of Consciousness" in the Journal of Consciousness Studies, coining the term "hard problem" to distinguish it from the "easy problems" (reportability, attention, access) that standard cognitive-scientific methods can explain by specifying a mechanism for the function. Any such functional explanation, Chalmers argues, leaves open why the performance of these functions is accompanied by experience at all — an explanatory gap (Levine's 1983 term, above) no physical account can close, since it remains conceivable that the same process occur without experience. Rather than deeming the problem intractable, Chalmers proposes treating experience as fundamental, governed by new "psychophysical" bridging laws, and sketches candidate principles (structural coherence, organizational invariance, a double-aspect theory of information) for what he calls "naturalistic dualism"; a footnote briefly dismisses Kripke-style a posteriori necessity as used by Hill, Levine, and Loar (above), a critique developed at length the following year in The Conscious Mind. The paper gives the hard-problem framing its public debut a year ahead of the book that would formalize the Type-A/Type-B distinction it structures.
1996	Classification introduced	David Chalmers publishes The Conscious Mind: In Search of a Fundamental Theory (Oxford University Press), arguing for property dualism via the conceivability-possibility argument: since it is conceivable that a world physically identical to ours lacks consciousness (a "zombie world"), and conceivability of this kind entails metaphysical possibility, facts about consciousness are further facts over and above the physical facts. In a footnote addressing physicalist responses that grant the conceivability of zombies but deny it shows anything, since psychophysical identities could be true as a posteriori necessities in the manner of Kripke's heat/molecular-motion case, Chalmers sets out the a priori/a posteriori entailment distinction this timeline is built around: such a view, he says, "must be combined with an appeal to strong metaphysical necessity in supporting the link between biochemistry and consciousness, inheriting all the problems with that view," and he names David Lewis and David Armstrong (1968, 1972, above) as exemplars of philosophers committed to it. Chalmers does not yet use the labels "Type-A" and "Type-B materialism" here; he introduces that explicit terminology only in his 1999 reply to critics (below), prompted directly by the responses of Hill and McLaughlin (1999, below) and others to this book.
1997	Publication	Christopher Hill publishes "Imaginability, Conceivability, Possibility and the Mind-Body Problem" in Philosophical Studies, answering Kripke's modal argument against type-materialism with two explanations of its apparent force: first, developing a footnote of Nagel's, that intuitions of disembodiment arise from splicing together a sympathetic image of a mental state with a perceptual image of the absence of its brain state, a general "image-splicing" mechanism independently known to produce false separability intuitions (e.g. heat from molecular motion); second, that the faculty of conception produces the same intuitions because sensory concepts like pain and neuroscientific concepts like C-fiber stimulation serve different classificatory roles and so lack any a priori tie, which on its own does not show the concepts fail to corefer.
2018	Publication	Janet Levin publishes "Once More Unto the Breach: Type B Physicalism, Phenomenal Concepts, and the Epistemic Gap" in the Australasian Journal of Philosophy, arguing that physicalists have underestimated the resources of Type-B physicalism and retreated too quickly to less promising defenses. Levin defends a Loar-style recognitional-demonstrative account of phenomenal concepts against Philip Goff's "revelation" challenge (that a phenomenal concept must be transparent, translucent, or opaque with respect to its referent's essence, with none of the options available to a physicalist realist about consciousness), arguing the physicalist can meet this challenge without conceding the gap is anything more than epistemic.
2020	Publication	Daniel Stoljar publishes "The Epistemic Approach to the Problem of Consciousness" in Uriah Kriegel's edited The Oxford Handbook of the Philosophy of Consciousness, restating his epistemic view that explanatory limitations regarding consciousness reflect contingent ignorance of relevant physical facts rather than any metaphysical gap — a view closer to Type-C materialism and mysterianism than to the mainstream phenomenal-concept-strategy form of Type-B physicalism.
2020	Publication	Joseph Levine publishes "A Posteriori Physicalism: Type-B Materialism and the Explanatory Gap" as Chapter 18 in Uriah Kriegel's edited The Oxford Handbook of the Philosophy of Consciousness (pp. 387–404; published online 9 July 2020), revisiting the explanatory gap he named in his own 1983 paper (above) and arguing for a notable shift in position: whereas Levine (1998, 2001) had held that the conceivability of zombies reveals a genuine explanatory gap between the physical and the mental but that there remained good reason to adopt materialism regardless, this chapter sides with the more skeptical view of Levine (2014), arguing that the persistence of the explanatory gap itself licenses an inference to the best explanation — namely, that the gap's existence and persistence is best explained by there being a genuine underlying metaphysical gap between the physical and the phenomenal, not merely an epistemic one. Levine lays out and rejects the standard argument that materialism must be abandoned simply because zombies are conceivable, and also rejects Type-B materialists' related claim that the explanatory gap poses no real problem for physicalism, arguing that the inference to the best explanation survives this defense.
2021	Publication	American philosopher Helen Yetter-Chappell publishes "Dissolving Type-B Physicalism" in Philosophical Perspectives, arguing directly against Type-B physicalism rather than merely defending Type-A: she constructs a hypothetical agent ("Mary*") who has complete general physical knowledge, possesses genuine phenomenal concepts (meeting the constitutional theory's own criteria, as developed by Katalin Balog and others), and yet is psychologically "wired" to immediately recognize that her phenomenal concepts and physical concepts corefer — deriving phenomenal-physical identities a priori rather than a posteriori. Since physicalists must grant that such an agent's low-level cognitive architecture is a complete physical description of a possible mind, and that this agent would find no epistemic gap, Yetter-Chappell concludes there is no ideal epistemic gap between the phenomenal and the physical: humans find such a gap only because of a contingent, seemingly non-adaptive quirk of human psychology, not because phenomenal concepts are intrinsically disconnected from physical concepts. She extends the argument beyond the constitutional theory to indexical (Perry 2001) and direct reference (Levin 2007, Tye 2003) theories of phenomenal concepts, arguing all three fail to secure an ideal gap for the same underlying reason. The paper's positive upshot is a new "Type-A phenomenal concept strategy" that keeps the intuitive payoffs Type-B physicalists wanted (that Mary would be fooled by a blue banana, that zombies would still seem conceivable to us) while denying that any of this reflects a genuine metaphysical or ideal epistemic gap.
2022 (June 16)	Publication	Philosopher David Chalmers, interviewed by Robert Lawrence Kuhn for the Closer to Truth series, walks a popular audience through his own Type-A/Type-B/Type-C classification of materialism (introduced in 1999, above) before rejecting all three: he characterizes Type-A materialism as denying that consciousness exists at all (eliminativism), restates Type-B materialism as holding that consciousness just is a brain process even though our concepts of the two remain distinct, and describes Type-C materialism as treating the hard problem as a currently unsolved but eventually science-resolvable puzzle. Having rejected each variety of materialism, Chalmers argues that consciousness should instead be treated as a fundamental category of reality in its own right, sketching further possibilities on which consciousness is either a causally inert epiphenomenon or has genuine causal impact on the physical world, possibly mediated by quantum mechanics.
2023	Publication	Marcelino Botin (University of Barcelona) publishes "Russellian Physicalists get our phenomenal concepts wrong" in Philosophical Studies (Vol. 180, pp. 1829–1848), defending Type-B physicalism by contrasting its resources with those of the rising alternative, Russellian physicalism (RP). Botin applies Philip Goff's 2011 "revelation challenge" — the argument that a phenomenal concept must be transparent, translucent, or opaque with respect to its referent's essential properties, and that none of these options is available to a physicalist who also wants realism about consciousness — to both views, and argues it is survivable for Type-B physicalism (which can hold that phenomenal concepts are translucent, revealing the essence of experience but only under a phenomenal rather than physical characterization, per Katalin Balog's constitutional account) but fatal for Russellian physicalism, which is committed to the further claim that phenomenal-physical identities are knowable a priori given a complete account of intrinsic physical properties — a commitment the phenomenal concept strategy's account of conceptual isolation cannot satisfy. Botin further argues that Type-B physicalism does not need to borrow any of Russellian physicalism's machinery (postulating inscrutable intrinsic properties) to explain the hallmark features attributed to consciousness — ineffability, irreducibility, and (apparent) intrinsicness — since the phenomenal concept strategy already accounts for these via conceptual isolation and constitutional reference.
2024	Publication	Christopher Devlin Brown and David Papineau publish "Illusionism and A Posteriori Physicalism; No Fact of the Matter" in the Journal of Consciousness Studies, arguing that the apparent dispute between illusionism (the view that phenomenal consciousness, in the strong sense implying non-physical properties, does not exist) and a posteriori (Type-B) physicalism is merely a definitional dispute rather than a substantive disagreement. The authors argue that whether to treat "phenomenal consciousness" as a concept to be eliminated (per illusionism) or reconstructed with a weaker, physically acceptable meaning (per Type-B physicalism, following Papineau's own long-standing phenomenal concept strategy) is a judgment call comparable to other cases in the history of science and philosophy where a term either survived with a revised meaning (as with "planet" or "star") or was abandoned outright (as with "phlogiston" or "witch") once its originally assumed properties turned out not to exist. On their view, both camps ultimately agree on the underlying physical facts and disagree only about which linguistic and conceptual packaging is most useful.
2024	Publication	Marcelino Botin publishes "A posteriori Russellian physicalism: a new solution to the meta-problem of consciousness" in The Philosophical Quarterly (published online 22 October 2024), extending his 2023 critique of Russellian physicalism (above) into a positive proposal. Botin argues that neither of the two leading physicalist realist positions — orthodox Russellian physicalism nor Type-B physicalism — can adequately solve Chalmers' "meta-problem of consciousness" (the problem of explaining why we believe there is a hard problem of consciousness in the first place). He then introduces "a posteriori Russellian physicalism," a hybrid view combining Russellian monism's appeal to intrinsic physical properties with the phenomenal concept strategy characteristic of Type-B physicalism, and argues this hybrid overcomes the specific difficulties that beset each view taken alone. Botin's stated aim is to show that physicalists need not retreat to illusionism, since a physicalist realist solution to the meta-problem remains available via this Russellian phenomenal-concept-strategy hybrid.
2024 (September 1)	Publication	Will Moorfoot publishes "Type-R physicalism" in Philosophical Psychology (Vol. 39, Issue 2, pp. 292–317; received 20 Oct 2023, accepted 18 Aug 2024, published online 1 Sep 2024), introducing a category of physicalism distinct from Chalmers' Type-A, Type-B, and Type-C: the "reconciliatory response" to the conceivability argument, which grants that zombies are metaphysically possible but denies this refutes physicalism. Moorfoot develops "stochastic ground physicalism" (SGP) as a concrete version of Type-R physicalism, on which phenomenal facts are stochastically (rather than deterministically) grounded in physical facts — the physical facts fix a probability distribution over possible phenomenal outcomes without necessitating any single one, which lets phenomenal facts count as physical (satisfying "Physicality") while remaining modally separable from their physical grounds (satisfying the metaphysical possibility of zombies). Moorfoot explicitly contrasts Type-R with Type-B: whereas Type-B physicalists reject the entailment from conceivability to possibility by positing "strong necessities" and accepting a resulting "modal dualism" between logical and metaphysical modality, Type-R physicalists reject the conceivability argument's final premise instead (that zombie-possibility entails physicalism's falsity), letting them jointly accept a thick, non-functionalist conception of consciousness, the transparency of phenomenal concepts, and full modal rationalism — a combination of resources Moorfoot argues is unavailable to Type-A, Type-B, or Type-C physicalists individually.
2025	Publication	Peng Zhou (Chinese University of Hong Kong) publishes "Indicating without revealing: a posteriori physicalism and the new challenge" in Synthese (Vol. 206, article no. 91; received 25 Nov 2024, accepted 7 Jul 2025, published online 6 Aug 2025), addressing Philip Goff's "New Challenge" against a posteriori (Type-B) physicalism, which argues that Type-B physicalism cannot do justice to the intuition that phenomenal concepts have a substantial, essence-revealing content without thereby violating the a posteriori status of phenomenal-physical identity claims. After surveying and rejecting the three existing Type-B responses to Goff's argument — radical opacity (Papineau, McLaughlin), soft translucency (Levin, Diaz-Leon, Schroer), and dual carving (including recent defenses by Janet Levin (2019, above) and D. Gene Witmer) — Zhou proposes a novel reply: phenomenal concepts have substantial contents in the sense of being "essence-indicating" (providing conditions that would partly or fully constitute the referent's essence) without their reference being determined by satisfaction of those conditions, so that a phenomenal concept can refer to a physical/functional property even though that property does not satisfy the concept's phenomenal content. Zhou acknowledges that this "essence-indicating without essence-revealing" solution places Type-B physicalism close to illusionism, potentially supporting a "unification thesis" recently argued for by Christopher Devlin Brown and David Papineau (2024, above/below) and by Takuya Niikawa (2021), on which the disagreement between a posteriori physicalism and illusionism is largely terminological.
2026 (April 8)	Publication	Philosopher and science communicator Phil Halper (Skydivephil) releases the documentary-style video "Consciousness: Philosophers & Neuroscientists Defend Physicalism" on YouTube, featuring interviews with philosophers and neuroscientists making the case for physicalism, including Robert Kirk and Frank Jackson — historically influential sources of anti-physicalist arguments (Kirk's zombie/conceivability arguments and Jackson's 1982 knowledge argument, above) — both now presented as having "turned to physicalism," alongside Ned Block, David Papineau, Richard Brown, Ken Williford, and neuroscientists Karl Friston, Anil Seth, and Mark Solms. The video addresses standard anti-physicalist arguments (the causal closure argument for physicalism, the zombie argument, Mary's Room, the hard problem, and empirical challenges from near-death and out-of-body experiences) and includes a segment specifically on "the revelation thesis," directly engaging the Type-B-relevant "New Challenge" debate (Goff, Zhou 2025, above) about whether phenomenal concepts reveal the essence of their referents.
2026 (June 28)	Publication	American computer scientist and philosopher Jessica Taylor ("jessicata") publishes "Powerful qualities in type B physicalism" on LessWrong, arguing that Type-B physicalism combines naturally with John Heil's "powerful qualities" (PQ) theory of properties, on which every intrinsic property is simultaneously qualitative and dispositional (redness just is a light-reflectance disposition, viewed under a different mode of presentation) rather than qualities and dispositions being separate "aspects." Taylor argues this pairing de-ad-hocs the phenomenal concept strategy by treating phenomenal concepts as a special case of the broader category of qualitative concepts, rather than sui generis "direct demonstrative"/"quotational" concepts per Katalin Balog and others; motivates treating "phenomenally conscious" as a secondary qualitative concept co-referring with a dispositional concept such as Ned Block's "access consciousness," supporting unconscious qualities (neural states that would be qualia-bearing if attended to, but currently aren't); and draws a structural parallel to debates in the metaphysics of perception, via a thought experiment about "direct realist" philosophers who locate color in external objects and face an equivalent explanatory-gap debate. Taylor credits Henry Taylor (2017, 2024) with first noting this concordance, and contrasts the view with Russellian monism/panqualityism, illusionism, and analytic functionalism.
2026	Publication	Will Moorfoot publishes "Type-R physicalism" in Philosophical Psychology (Vol. 39, Issue 2, pp. 292–317; received 20 Oct 2023, accepted 18 Aug 2024, published online 1 Sep 2024), introducing a category of physicalism distinct from Chalmers' Type-A, Type-B, and Type-C: the "reconciliatory response" to the conceivability argument, which grants that zombies are metaphysically possible but denies this refutes physicalism. Moorfoot develops "stochastic ground physicalism" (SGP) as a concrete version of Type-R physicalism, on which phenomenal facts are stochastically (rather than deterministically) grounded in physical facts — the physical facts fix a probability distribution over possible phenomenal outcomes without necessitating any single one, which lets phenomenal facts count as physical (satisfying "Physicality") while remaining modally separable from their physical grounds (satisfying the metaphysical possibility of zombies). Moorfoot explicitly contrasts Type-R with Type-B: whereas Type-B physicalists reject the entailment from conceivability to possibility by positing "strong necessities" and accepting a resulting "modal dualism" between logical and metaphysical modality, Type-R physicalists reject the conceivability argument's final premise instead (that zombie-possibility entails physicalism's falsity), letting them jointly accept a thick, non-functionalist conception of consciousness, the transparency of phenomenal concepts, and full modal rationalism — a combination of resources Moorfoot argues is unavailable to Type-A, Type-B, or Type-C physicalists individually.

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• Timeline of type A physicalism
• Timeline of consciousness research
• Timeline of David Chalmers' work on consciousness