Adjudicating the definitional dispute over 'phenomenal consciousness' between illusionism and anti-illusionism

Illusionism claims that phenomenal consciousness does not exist but merely seems to; anti-illusionists hold that its existence cannot be reasonably doubted. Niikawa (2021) argued that this radical disagreement is fundamentally terminological: illusionists operate with a 'dubitable' sense of 'phenomenal consciousness' — one incorporating the thought-experiment-motivated metaphysical claim that phenomenal feelings differ from physical and functional properties — while anti-illusionists operate with an 'indubitable' sense fixed by the what-it-is-like characterization. The paper explicitly leaves open which sense the field should adopt, calls settling this urgent, and raises considerations for adjudication, naming as examples historical authenticity, academic productivity, and association with the ordinary concept of consciousness. The definitional question remains actively contested in the 2022–2026 follow-up literature — by illusionist loaded definitions, realist neutral definitions, Kammerer's 'explicitly innocent but implicitly loaded' account, and Brown and Papineau's semantic-indeterminacy deflationism — with no accepted adjudication. The problem is to resolve it.

Background

Phenomenal consciousness is the subjective, experiential aspect of mental life. Following Nagel's (1974) famous bat example, philosophers standardly gloss it with the 'what it is like' locution: a mental state is phenomenally conscious when there is something it is like to be in it. Illusionism (Frankish 2012) is the thesis that phenomenal consciousness does not exist: nothing has the properties introspection seems to reveal — the qualia, i.e. the purported intrinsic, qualitative features of experience — although we are strongly inclined to think that something does. Anti-illusionists (phenomenal realists) find this absurd: whatever else is true, that experience involves such felt qualities is, they hold, not something anyone could reasonably doubt. The persistence and vehemence of this standoff — adjacent to the notorious 'hard problem' of explaining why physical processing is accompanied by subjective experience at all — is itself a puzzle about the debate's structure.

Niikawa (2021) asked where the disagreement really lies. His method was to distinguish senses of 'phenomenal consciousness' by whether the existence of the sense's referent can be reasonably doubted: a sense is dubitable if reasonable doubt is possible, indubitable if not, and grey if that is precisely what is contested. He then examined the main characterizations of the term found in the literature: the what-it-is-like locution itself; inner ostension (attending inward to one's current experience and using it to fix what a term such as 'phenomenal migraine' refers to, somewhat as pointing at an object fixes a public term's reference); the classical thought experiments — philosophical zombies (creatures functionally identical to us but lacking experience), inverted qualia (two subjects functionally identical but with systematically swapped colour experiences), and Mary's room (a scientist raised without colour experience who seems to learn something new when she first sees colour); appeals to scientific knowledge of secondary properties (properties such as colour, whose appearance-based treatment in colour science seems to leave their experiential side scientifically elusive); theoretical claims such as ineffability and intrinsicness; and appearance/reality collapse (the idea that for experience, unlike for external objects, seeming to have it guarantees having it).

Two clauses matter most. The metaphysical thought-experiment clause (M-TE) counts a state $X$ of subject $S$ as phenomenally conscious when $X$ is constitutively such that there is some way $S$ feels in virtue of being in $X$, and that feeling is different from $S$'s physical and functional properties; the epistemic variant (E-TE) requires only that the feeling seems to be different from those properties. Niikawa argued that the what-it-is-like, inner-ostension, E-TE, and secondary-property senses are indubitable; the M-TE and theoretical-claim senses dubitable; and the appearance/reality sense grey only because it inherits the terminological controversy. His terminological hypothesis is that illusionists adopt a dubitable sense (one including M-TE), anti-illusionists an indubitable sense, and there is no single sense in which the two camps genuinely disagree about whether its referent can be reasonably doubted. On this diagnosis the real question of the debate is whether M-TE belongs in the definition of 'phenomenal consciousness' at all. (The taxonomy presupposes two assumptions Niikawa flags: that what-it-is-like talk is an ordinary rather than purely technical locution — a question also pursued in the semantics literature by Stoljar (2016) — and that phenomenal consciousness is the highest determinable of experiences, i.e. the most general property under which particular experiences fall as determinates, in the way 'being colored' subsumes 'being scarlet'.)

Crucially, Niikawa stresses that 'terminological' does not mean trivial or easy. Deciding which sense the field should adopt requires weighing a range of considerations, of which Niikawa names three examples: (1) historical authenticity — which sense matches how the notion was actually introduced and spread, a history entangled with the very thought experiments that motivate M-TE; (2) academic productivity — which sense best serves consciousness studies; and (3) the term's association with the ordinary concept of consciousness. He calls settling this terminological debate urgent, but does not settle it, and he explicitly sets aside auxiliary issues (whether zombies are really conceivable, contested readings of colour science, and the general epistemic standard for when doubt is reasonable, on which he defers to Thagard (2004)).

Problem Statement

Which sense of 'phenomenal consciousness' should be adopted in consciousness studies — the dubitable sense that includes the metaphysical thought-experiment clause M-TE (on which a phenomenally conscious state of subject $S$ is one such that there is some way $S$ feels in virtue of being in it, and this feeling is different from $S$'s physical and functional properties), or an M-TE-free indubitable sense (the what-it-is-like characterization, with or without inner ostension, and with or without conjoined clauses such as the epistemic thought-experiment reading E-TE, on which that way of feeling merely seems to be different from the subject's physical and functional properties, or the scientific-knowledge characterization, on which phenomenal consciousness is fixed as what contains experiential secondary qualities such as colour, taste, and smell), or some other specified precisification policy for the term? Progress in adjacent debates (zombie conceivability, readings of colour science, the semantics of what-it-is-like talk and of inner ostension) may be invoked as inputs, but those debates are not themselves required deliverables.

References

1. Niikawa, Takuya (2021). 'Illusionism and definitions of phenomenal consciousness.' Philosophical Studies 178(1): 1–21. DOI: 10.1007/s11098-020-01418-x. https://doi.org/10.1007/s11098-020-01418-x (first published online 23 January 2020; open-access accepted manuscript at https://da.lib.kobe-u.ac.jp/da/kernel/90007807/90007807.pdf)
2. Nagel, Thomas (1974). 'What Is It Like to Be a Bat?' The Philosophical Review 83(4): 435–450. DOI: 10.2307/2183914. https://doi.org/10.2307/2183914
3. Frankish, Keith (2012). 'Quining diet qualia.' Consciousness and Cognition 21(2): 667–676. DOI: 10.1016/j.concog.2011.04.001. https://doi.org/10.1016/j.concog.2011.04.001
4. Stoljar, Daniel (2016). 'The Semantics of "What it's like" and the Nature of Consciousness.' Mind 125(500): 1161–1198. DOI: 10.1093/mind/fzv179. https://doi.org/10.1093/mind/fzv179
5. Thagard, Paul (2004). 'What Is Doubt and When Is It Reasonable?' Canadian Journal of Philosophy, Supplementary Volume 30: 391–406. DOI: 10.1080/00455091.2004.10717611. https://doi.org/10.1080/00455091.2004.10717611
6. Kammerer, François (2022). 'How can you be so sure? Illusionism and the obviousness of phenomenal consciousness.' Philosophical Studies 179(9): 2845–2867. DOI: 10.1007/s11098-022-01804-7. https://doi.org/10.1007/s11098-022-01804-7
7. Brown, Christopher D. and Papineau, David (2024). 'Illusionism and a Posteriori Physicalism: No Fact of the Matter.' Journal of Consciousness Studies 31(7): 7–27. DOI: 10.53765/20512201.31.7.007. https://doi.org/10.53765/20512201.31.7.007
8. Wyrwa, Michał (2025). 'A More Ordinary Ordinary View of the "What it is Like"-Talk.' Philosophia 53(2): 743–768. DOI: 10.1007/s11406-025-00865-2. https://doi.org/10.1007/s11406-025-00865-2
9. Kammerer, François (2025). 'Defining consciousness and denying its existence. Sailing between Charybdis and Scylla.' Philosophical Studies 182(2): 541–565. DOI: 10.1007/s11098-025-02285-0. https://doi.org/10.1007/s11098-025-02285-0
10. Kammerer, François (2026). House of Mirrors: The Illusion of Phenomenal Consciousness. New York: Oxford University Press. DOI: 10.1093/9780197840283.001.0001. https://doi.org/10.1093/9780197840283.001.0001 (open-access PDF hosted by the author at https://www.francoiskammerer.com/wp-content/uploads/2026/06/Kammerer-2026-House-of-Mirrors.-The-Illusion-of-Phenomenal-Consciousness.pdf)