Kripke’s Modal Argument Against Mind–Brain Identity

Saul Kripke’s theory of phenomenal consciousness is not a particular psychophysical model but a modal-metaphysical argument that conscious states such as pain are essentially characterized by their felt phenomenal character, so that any attempted identity between such states and brain states would have to be necessary if true, yet appears not to be necessary in a way that cannot be explained away as in ordinary scientific identities. As such, it remains the most influential modal argument against the physicalist reduction of phenomenal consciousness (Kripke, 1971, 1980; Levine, 1983; Chalmers, 1996).

Using the arguments of Naming and Necessity—rigid designation, the necessity of identity, and the category of the “necessary a posteriori”—he argued that if a conscious state such as pain were identical to a brain state such as C-fiber firing, that identity would have to be metaphysically necessary, yet the identity seems contingent (we can apparently conceive of pain without C-fibers and C-fibers without pain), and, uniquely among scientific identities, this appearance of contingency cannot be explained away as an illusion, because a state's feeling like pain is not a contingent mode of presentation of something else but is constitutive of pain itself; the identity theory is therefore false (Kripke, 1980; Speaks, 2018).

Rigid Designation and Necessary Identity

Kripke’s argument about consciousness depends on his broader theory of reference and modality. A rigid designator refers to the same object in every possible world in which that object exists; this idea has consequences for metaphysics because whether the terms in an identity statement are rigid helps determine whether the identity, if true, is necessary or contingent. Kripke’s paradigm examples of rigid designators are proper names and natural-kind terms: “Hesperus (the evening star, Venus) = Phosphorus (the morning star, Venus)” and “water = H₂O” are discovered empirically, but if the two terms rigidly designate the same entity or kind, the identity is necessary if true (Kripke, 1980). By way of contrast, definite descriptions ("the inventor of bifocals") typically are not (Kripke, 1980). The striking result is the necessary a posteriori: truths that can be known only empirically, but are metaphysically necessary.

Applied to mind, this framework changes the status of psychophysical identity claims. If “pain” and “C-fiber firing,” or more generally “pain” and some neural state N, rigidly designate their referents, then “pain = N” cannot be contingently true. It must be necessary if true at all. Kripke’s point is not that science cannot discover surprising identities; rather, it is that genuine identities do not hold merely in the actual world. If pain really is a certain neural state, then in every possible world in which that pain exists, that neural state must exist as well, and vice versa.

The Illusion of Contingency and Why Science Usually Dissolves It

Such necessary identities nonetheless seem contingent, and Kripke's account of why is what makes his mind–body argument work. It seems we could discover that Hesperus was not Phosphorus, or that water was not H₂O. Kripke explains this appearance away: what we are really conceiving is a qualitatively similar situation in which some other thing plays the superficial identifying role—a world where a body other than Venus appears in the evening sky, or where a watery-seeming stuff is not H₂O. The apparent contingency is thus reinterpreted as a genuine possibility misdescribed: we confuse the necessary identity with a distinct, contingent claim about how things are picked out (Kripke, 1980; Speaks, 2018). For ordinary theoretical identifications, the mode of presentation (how a thing appears) is distinct from the thing itself, so there is room for this reinterpretation.

Why Pain is Different

Kripke's decisive claim is that this reinterpretation is unavailable for psychophysical identities. Suppose "pain = C-fiber firing" were a necessary a posteriori truth. To explain away its apparent contingency, the physicalist would need to say that when we imagine pain without C-fiber firing, we are really imagining a situation that merely feels like pain without being pain—just as we imagine something that merely looks watery without being water. But this move fails, because in the case of a sensation there is no distinction between appearance and reality: for something to feel like pain is for it to be pain (Kripke, 1980). Pain is picked out by an essential property—its immediately felt phenomenal character—rather than by a contingent mode of presentation. Hence there is no qualitatively identical but distinct scenario to which the illusion of contingency can be redirected. If pain and C-fiber firing can even seem to come apart, then, given the necessity of identity, they genuinely can come apart, and so are not identical. The same reasoning generalizes against type–type, token, and person–body identity theories.

Significance, Influence, Objections

Kripke's argument reoriented late-twentieth-century philosophy of mind. Though he presented it as a renovation of the Cartesian conceivability argument (Descartes, 1641), its novelty was to show that the standard materialist reply—dismissing anti-identity intuitions as illusions of contingency of the familiar scientific kind—cannot be applied to phenomenal states, precisely because such states lack the appearance/reality gap that other scientific identities exploit. This turn on the special epistemology of the phenomenal is the direct ancestor of David Chalmers's conceivability (zombie) argument and two-dimensional framing of the hard problem (Chalmers, 1996), and it sharpened the explanatory-gap literature by locating the difficulty in the constitutive tie between a phenomenal state and its felt character.

The argument, not surprisingly, is also widely contested. Critics—most influentially Scott Soames—object that Kripke conflates epistemic with metaphysical possibility: that pain and C-fiber firing can seem to come apart shows only an epistemic gap, and physicalists (via a posteriori or "type-B" materialism) can hold the identity metaphysically necessary while conceding it is not knowable a priori, so the apparent contingency need not be a genuine one (Soames, 2015; Hill & McLaughlin, 1999). Others question whether "pain" and "C-fiber firing" are both genuinely rigid, or whether the felt/physical distinction is sufficiently robust. Whatever its ultimate outcome, Kripke's argument remains a fixed reference point in debates over whether phenomenal consciousness can be identical to, or necessitated by, the physical (Stoljar, 2021).

To be clear, what Kripke's argument does and does not establish is worth stating precisely. The argument, if sound, refutes identity (pain ≠ C-fiber firing); it does not, cannot, by itself establish dualism or refute all physicalism, since correlation, supervenience, or realization could survive (Searle and others press exactly this). Kripke himself was cautious and did not build a positive dualist system on it. Kripke’s target is the mind-brain identity theory specifically; it should not be read as "Kripke refuted materialism."

References

Chalmers, D. J. (1996). The Conscious Mind: In Search of a Fundamental Theory. Oxford University Press.

Descartes, R. (1641). Meditations on First Philosophy. Paris: Michel Soly.

Hill, C. S., & McLaughlin, B. P. (1999). There are fewer things in reality than are dreamt of in Chalmers's philosophy. Philosophy and Phenomenological Research, 59(2), 445–454.

Kripke, S. A. (1980). Naming and Necessity. Harvard University Press.

Soames, S. (2015). Kripke on mind–body identity. In Reference and Existence discussion papers / USC. (Also in A. Berger, Ed., Saul Kripke, Cambridge University Press, 2011.)

Speaks, J. (2018). Kripke's argument against materialism [Course handout, PHIL 30304]. University of Notre Dame.

Stoljar, D. (2021). Physicalism. In The Stanford Encyclopedia of Philosophy. Stanford University.