Modal Rationalism and the Mind-Body Problem

David J. Chalmers

Department of Philosophy
University of Arizona
Tucson, AZ 85721

[This is a detailed outline for a talk I've given a few times recently. I hope it is comprehensible, at least to philosophers who have read The Conscious Mind. This started life as a self-contained version of material from my response (Materialism and the Metaphysics of Modality) in the Philosophy and Phenomenological Research symposium on my book, but it contains a fair bit of material that isn't in that response. Much of this material is being further developed in work in progress, which may see the light of day either as a series of papers or as a short book (working title: Mind and Modality). See also my Princeton lectures on Mind and Modality, which contain this material as a proper part.]


The phenomenal facts are not entailed a priori by the physical facts. (See The Conscious Mind, chapter 3.)


(i) Type-A materialism: Deny the epistemic intuitions, and assert a priori entailment (eliminativism, analytic functionalism, ...). Not considered here.

(ii) Type-B materialism: Accept the epistemic intuitions, but hold that there is an a posteriori necessary entailment from physical to phenomenal. Physical:phenomenal as water:H2O or Hesperus:Phosphorus. Conceivability doesn't imply possibility.


This analysis explains a posteriori necessity in terms of two-dimensional semantics, over one space of possible worlds. Every statement S has two intensions, a primary and secondary intension. (See The Conscious Mind, section 2.4, and The Components of Content.)

Primary intension of S:

Secondary intension of S:

S is "necessary" -> S has a necessary secondary intension. ("subjunctive necessity").

S is a priori -> S has a necessary primary intension ("indicative necessity").

S is a posteriori necessary -> S has a contingent primary intension and a necessary secondary intension.

Supports modal rationalism: when S is conceivable (i.e. not-S is not a priori), S describes a possible world, at least according to its primary intension (although perhaps not according to its secondary intension). We retain a priori access to the space of possible worlds.


[This is a crisper version of the argument in The Conscious Mind, section 4.2]

Premise 1: The physical facts don't a priori entail all the facts.

Premise 2: When S is a posteriori, S has a contingent primary intension.

Premise 3: If materialism is true, the physical facts necessitate all true propositions.

4: P->Q is a posteriori. [P=physical facts, Q = a phenomenal fact] (from 1)

5: P->Q has a contingent primary intension (from 2, 4)

6: P'->Q' is contingent [P',Q' = primary intensions of P,Q] (from 5)

7: Materialism is false (from 3, 6; see loophole 2)

Loophole 1: Primary intensions are indexical; Q' is a centered proposition.

Response 1: Adding indexical information to P doesn't help epistemic arguments. P&I->Q is still a posteriori, P'->(Q' at I) is contingent

Loophole 2: Maybe microphysical facts have different PI/SI, so P necessitates phenomenal facts but P' does not.

Response 2: This leads to "panprotopsychism", with fundamental protophenomenal properties as the categorical basis of microphysical dispositions; a possible position, but more in the spirit of dualism than materialism.

UPSHOT: a posteriori necessity (at least of the usual variety) can't save materialism.

If it is conceivable that there are zombies, then the primary intension of "There are zombies" describes at least one possible world, and materialism is false. (Conceivability implies possibility at the level of worlds, if not at the level of statements.)


Only way out for the type-B materialist: deny the 2-D analysis of a posteriori psychophysical necessities. The 2-D analysis works for the standard Kripkean a posteriori necessities, but not for psychophysical necessities.

Must deny premise 2. Psychophysical necessities, although a posteriori, have a necessary primary intension.

Must deny modal rationalism: Zombies, although conceivable, are strongly metaphysically impossible: there is no possible world satisfying even the primary intension of "There are zombies". The conceivable world corresponds to no possible world at all.


Crucial issue: Are there strong necessities? ["Strong metaphysical necessities", in The Conscious Mind, section 4.2.]

Strong necessity = an a posteriori necessity with a necessary primary intension.

If S is a posteriori, there is some conceivable world W such that if W is actual, S turns out to be true. If S is a standard a posteriori necessity, W is possible. If S is a strong necessity, W is not possible.


Distinguish three independent dimensions in notions of conceivability (of a statement S):

Prima facie vs. ideal conceivability (conceivability on first appearances, vs. on ideal rational reflection).

Positive vs. negative conceivability (conceivability of a situation in which S seems true, vs. absence of any apparent contradixction in S).

Primary vs. secondary conceivability (conceivability when considered as actual vs. when considered as counterfactual).

Standard worries about conceivability vs. possibility, translated into this framework:

The plausible thesis: ideal primary positive conceivability of S implies possibility (of a world satisfying S's primary intension).

And just maybe: ideal primary negative conceivability of S implies possibility (ditto): but see sideline 2 below.


S is a priori if it is rationally knowable (independent of experience).

S is a posteriori if there is a conceivable world in which it turns out false.

S is inscrutable if it is neither a priori nor a posteriori (as defined here): not rationally knowable, but not false in any conceivable worlds. i.e., S is ideally negatively conceivable, but not ideally positively conceivable.

Deep issue: are there any inscrutable truths?


I hold: there are no inscrutable truths. But I will not argue that case here. What matters for the mind-body issue are strong necessities, not inscrutable necessities (as zombies are positively conceivable).

So the thesis I will argue for here is not

STRONG MODAL RATIONALISM: S is a priori iff it has a necessary primary intension

(although I believe it), but rather

WEAK MODAL RATIONALISM: S is a posteriori iff it has a contingent primary intension.

Strong modal rationalism requires that we rule out both strong necessities and inscrutable truths. Weak modal rationalism requires that we rule out only strong necessities.


Possible examples of strong necessities:

[For more discussion see my PPR response, section 3.3.]


In The Conscious Mind, I say that strong necessities will be brute facts and inexplicable. (And are thus better seen as contingent fundamental laws.) Some "explanations" have been offered in response:

Response: All these "explanations" fail. (See my PPR response, section 3.4.)


Materialist might say: I still haven't ruled out the possibility that strong necessities exist. Perhaps strong necessities are sui generis and inexplicable? Or perhaps an explanation is waiting to be found?

Q: How to argue positively for modal rationalism? (So far, we've argued negatively by defeating counterexamples and arguments against it.)

A: Step by step...

(i) We need the modality of logically possible (= ideally conceivable) worlds. EVEN IF some of these worlds aren't "metaphysically possible", we need these worlds for most of the purposes that we need possible worlds in the first place.

Imagine that a "strong laws" view is true, and counternomic worlds are metaphysically impossible. (Or that a type-B materialist view is true, and zombie worlds are metaphysically impossible.) Still, we will need counternomic worlds and zombie worlds in some strong sense for the above purposes: to make sense of a scientist's thought processes, counterfactuals concerning counternomic scenarios, the differing meanings of nomically coextensive terms, and so on. Same for zombie worlds. (See my PPR response, section 3.2.)

(ii) There is no bar to the existence of logically possible worlds. If one doesn't want to just postulate them, they are easy to construct (to "ersatz" into existence), e.g. in terms of maximal a priori consistent sets of statements (with small twists to handle centering and two-dimensional semantics). They behave just as possible worlds should: one can tell coherent science fiction about them, one can semantically evaluate all our terms and statements in them, and one can consider them both as actual and counterfactual.

(iii) So: we need the logical modality and there is no bar to it. So insofar as there is reason to believe in modality at all, there is reason to believe in this modality.

(iv) From the logical modality, we can recover all modal phenomena in which we have reason to believe; and we can use it to explain everything which modality can plausibly explain. We can use it in explaining counterfactuals, the contents of thought, rational inference, the semantics of language, and so on. And with the help of two-dimensional semantics (plus nonmodal facts), we can use it to explain such "metaphysical" phenomena as a posteriori necessity, the concept/property distinction, and so on.

(v) So: one modal primitive (plus conceptual analysis plus nonmodal facts) gives us everything we have reason to believe in. And this modal primitive is tied constitutively to the rational notions (if it's not, we won't be able to explain them): hence, constitutive ties between rational and modal notions.

(vi) The believer in strong necessities, by contrast, must embrace a modal dualism: there are distinct primitive modalities of logical and metaphysical possibility. The metaphysically possible worlds are a subset of the logically possible worlds, and neither class is reducible to the other. One modality handles "rational" modal concepts, the other handles "metaphysical" modal concepts.

(vii) There is no reason to believe in such a distinct "metaphysical" modality. (There is a metaphysical modality, but it is just the logical modality.) There is nothing for a distinct metaphysical modality to explain; what is plausibly explainable is already explained. The only reason to believe in it is to justify certain antecedently held theoretical views (e.g. materialism); but this justification is entirely circular in this context. The distinct modality is a frictionless wheel.

(viii) We do not even have a distinct concept of metaphysical modality for the second primitive to answer to. The momentary impression of such a concept stems from a false and confused understanding of such ontic/epistemic distinctions as that between apriority and necessity, and that between concept and property. But all that is easily subsumed under modal monism with the help of two-dimensional semantics.

(ix) Ultimately, there is just one circle of modal concepts, including both the rational modal concepts (validity, rational entailment, a priority, conceivability) and the metaphysical modal concepts (possibility, necessity, property). The relation between these is subtle, but it's a confusion to postulate a new primitive modality: all our reasons for believing in the metaphysical modality are grounded in the rational modality in the first place. (Witness the use of conceivability arguments in establishing a posteriori necessity.)


One important loose end: to establish "pure" modal rationalism, we need to cross the bridge from a priority to necessity, most likely by equating a priority with necessity of primary intension. But this requires ruling out inscrutable truths. If we can't rule out such truths, we have distinct circles of "positive" and "negative" modal concepts (cf. positive and negative conceivability); so close yet so far! This isn't so crucial for the mind-body issue, but it's an important distinct project in its own right.


If the anti-materialism argument is accepted (so type-A and type-B materialism are rejected), we're left with about three positions to choose from. All might be called versions of "phenomenal fundamentalism" (it's arguable whether (iii) is dualism).