A Reductio of Naturalism

Keep Calm and Study PhysicsLet’s define Naturalism as the view that everything is either physical or causally dependent on the physical. On this definition, Naturalism encompasses both “hard naturalism” (strict reductive physicalism) and “soft naturalism” (which allows for some non-physical things such as minds, provided those non-physical things are causally dependent on physical things).

For completeness, let’s also define physical as a catch-all term for those entities and properties recognized by modern physics (subatomic particles, forces, etc.) or any reasonable refinement thereof (i.e., any refinement that doesn’t introduce radically different ontological categories). On this view, whatever is physical must be spatiotemporal.

I now offer a reductio ad absurdum of Naturalism, as defined above, which deduces the non-truth of Naturalism from its truth.

  1.  Naturalism is true. [assumption for reductio]
  2. If Naturalism is true, then Naturalism is possibly true.
  3. If Naturalism is possibly true, then, necessarily, Naturalism is possibly true.
  4. Necessarily, Naturalism is possibly true. [from 1, 2, 3]
  5. There is at least one necessary truth. [from 4]
  6. There is at least one necessarily true proposition. [from 5]
  7. Necessarily, if some proposition P is true, then P exists.
  8. If some proposition P is necessarily true, then P necessarily exists. [from 7]
  9. There is at least one necessarily existent proposition. [from 6, 8]
  10. There is something that does not exist contingently. [from 9]
  11. If Naturalism is true, then everything that exists, exists contingently.
  12. Not everything that exists, exists contingently. [from 10]
  13. Naturalism is not true. [from 11, 12]

Some commentary on each step of the argument:

2 should be beyond reasonable dispute. Whatever is actually the case must be possibly the case. If something is not possibly true, then it’s necessarily not true, and therefore not true. Anyone who denies this is beyond reason.

3 is based on a principle of modal logic (commonly known as axiom 5) according to which whatever is possible is necessarily possible. In other words, if something is possible in the actual world, it must be possible in every possible world, which is to say, it can’t be impossible in any possible world. Not all philosophers accept this principle, but I can’t see why a Naturalist would dispute this instance of it. It would be very odd to think that Naturalism is possible in the actual world, yet there are possible worlds in which Naturalism is impossible. Why would Naturalism be possible in some possible worlds but not in others?

4 follows deductively from 1, 2, and 3.

5 follows straightforwardly from 4. It’s hard to see why anyone would affirm 4 but deny 5. If 4 is true, then it’s a necessary truth, and therefore there is at least one necessary truth.

6 follows from 5 on the understanding that a proposition is by definition a primary truth-bearer, i.e., propositions are just those things that can bear truth-values (and bear them underivatively). Hence a truth just is a true proposition, and a necessary truth just is a necessarily true proposition. Note that 6 doesn’t assume anything about the metaphysical nature or structure of propositions, other than that they’re the kind of thing that can be true or false.

7 is grounded in the metaphysical principle that only existent things can possess properties. If S possesses the property P, then S exists. 7 is a specific instance of that principle, viz., a proposition must exist in order to possess the property of being true.

8 follows straightforwardly from 7. If a proposition is necessarily true, then it’s true in every possible world, and if a proposition must exist in order to be true, then it must exist in every possible world, which is just to say that it necessarily exists. (More technically: 8 follows from 7 via the distribution axiom of modal logic, also known as axiom K.)

9 follows deductively from 6 and 8.

10 obviously follows from 9, given that necessary existence is equivalent to non-contingent existence.

11 requires some explanation. Naturalism, as defined here, is the view that everything is either physical or causally dependent on the physical. By nature, physical things exist contingently. For any existent physical thing X (e.g., a subatomic particle), X need not have existed. The same goes for any composite of physical things: you can’t get a necessary existent by combining contingent existents. Likewise, anything that’s causally dependent on physical things must also exist contingently. Naturalism thus entails that everything that exists, exists contingently.

It’s important to note that even if some physical things were to exist eternally, they would still exist contingently. Eternal existence isn’t equivalent to necessary existence (neither does the former entail the latter). To confuse the two is to confuse temporal categories with modal categories. To exist eternally is to exist at every time in the actual world; to exist necessarily is to exist in every possible world.

12 follows deductively from 10, and 13 follows 11 and 12 by modus tollens.

All of the premises except 1 (i.e., 2, 3, 7, and 11) are necessarily true if true at all. 2 and 3 are based on widely accepted axioms of modal logic, 7 is grounded in a metaphysical principle about property possession, and 11 is a necessary consequence of Naturalism. Thus, if the argument is logically valid, 13 is a necessary implication of 1. Since 13 is also the negation of 1, it follows by reductio that 1 is false (i.e., Naturalism is not true).

Note that the argument doesn’t make use of the Principle of Sufficient Reason or any other principle characteristic of cosmological arguments (e.g., that every contingent existent requires a cause of its existence). The notion of causation features only in the explanation for 11.

I don’t expect any Naturalist to concede the argument. (Then again, maybe I’ll get lucky!) However, assuming the argument is logically valid, it would be interesting to know which of the premises (other than 1!) a Naturalist would be most willing to reject and why. Each of the premises could be disputed, but they’re not equally disputable, and each has a cost associated with denying it. What price is a Naturalist willing to pay?

7 Responses to A Reductio of Naturalism

  1. James Gibson

    Deny 11. Spacetime exists in every possible world. So spacetime is necessary. Spacetime is described by physics. So spacetime is physical. So there is a physical thing that exists in every possible world. So not every physical thing exists contingently if naturalism is true. (One might be able to run the same sort of argument with spacetime points, where there is at least one necessary spacetime point.)

    • Why would spacetime exist necessarily? That smacks of special pleading. Moreover, I’m not sure I understand what it would mean for spacetime to exist alone, in the absence of any spatiotemporal objects/universe. Spacetime is more like a property or mode of physical things, rather a physical thing itself.

      • James Gibson

        The point is similar to the Kantian thought that every perception depends on the concept of space and time (and substance), but it drops the idealism. Likewise, all substance or particulars depend on spacetime. (Didn’t Peter Strawson make this point?) If you think the naturalist would be right to share the thought that there had to be something rather than nothing, then the naturalist might look for some plausible candidates of what every contingent thing would depend upon. up So if spacetime is necessary for the existence of everything else and it depends on nothing else, isn’t that a plausible candidate for necessary existence? So I don’t think that appeal is obviously special pleading. It might be wrong, but not special pleading.

        The naturalists might also say in response, “we agreed with your definition of naturalism, but 11 just asserts that it follows that everything is contingent if naturalism is true. We signed up for naturalism, and 11 doesn’t follow from that definition.”

        One more point on behalf of the naturalist. The counter argument I opened with doesn’t imply that spacetime could exist in the absence of any spatiotemporal objects. It is noncommittal about that (purposely). The glass is full of water, but it could have lacked water. Now the glass lacks water, but it is still full of something else: air. Although neither’s presence is necessary, there must be something.

        • Thanks, James. That’s interesting. But I don’t think a Strawsonian argument (even a non-idealist version) could deliver the conclusion that spacetime exists necessarily (in the metaphysical sense). It only shows that spacetime is a necessary condition for certain cognitive operations or judgments (about substances, particulars, etc.). That gets you to a kind of transcendental necessity, but not a metaphysical necessity.

          I don’t think 11 merely asserts that everything is contingent if Naturalism is true. I explained how it follows from the given definition of Naturalism (which is a widely accepted one, I think).

          As for your last point, I’m not sure I follow it. But if you’re suggesting what I think you’re suggesting, I’d say it confuses “Necessarily something exists” with “Something necessarily exists”. To say that some spatiotemporal object must exist is not to say that there is a necessarily existent spatiotemporal object.

  2. Hi James (Anderson, though not to exclude the other :),
    I’m wondering if the heart of the argument turns on considerations about propositions, considerations that may be separable from certain premises you have here.

    One issue is whether propositions, in the relevant sense (underivative truth-bearers), could be physical or caused by the physical. If the answer is “no”, it seems that naturalism is threatened by the existence of truths, whether or not there are necessary ones, and whether or not naturalism implies that all existents are contingent. (Compare: naturalism is threatened by the existence of abstracta, as such.)

    Or suppose propositions could be physical. Maybe we don’t need the claim that naturalism implies that all existents are contingent. Perhaps we can get by with the claim that naturalism implies that Xs are contingent, where Xs comprehend all the kinds of physical things that could be propositions. I’m not sure what the best candidate for Xs would be on naturalism, but two ideas that come to mind are token brain states and token linguistic inscriptions – or something metaphysically dependent on such (e.g., types of brain states or inscriptions that are grounded in the tokens).

    Whatever Xs are, let P be a proposition that (1) naturalism implies is possibly true, and that (2) could only be true in a world devoid of Xs. (For example: the proposition that there are no sentient beings.) (1) and (2) would seem to be an incoherent set of features: if a world is devoid of Xs, it is devoid of propositions, including the proposition supposed to be true in the world. I haven’t spelled out the argument with the rigor you do, but I think the reasoning would turn on the same principles about getting from possible truths (e.g., “there are no sentient beings”) to necessary ones, and from truths to existing propositions.

    • I agree that the existence of propositions are a threat to naturalism on multiple fronts. Greg Welty and I get into some of the issues in our article on the argument for God from logic. However, here I’m trying to set up an argument that largely bypasses questions about what propositions would be (or how propositions would be grounded) in a naturalistic scheme.

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