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PSR for Naturalists

(Revised in light of Dr. Rizz's excellent comments)

It's commonly thought that a naturalist can't accept the Principle of Sufficient Reason (PSR), on the grounds that (i) she thereby commits herself to the existence of a metaphysically necessary being, and that (ii) this is incompatible with any plausible version of naturalism. I have my doubts about (ii), but here I want to focus on (i). For it seems to me that (i) is false. To be a tad more precise: there is a plausible version of PSR, the acceptance of which doesn't thereby require one to admit a metaphysically necessary being into one's ontology. William Lane Craig has recently defended a version of PSR as a component of his favored formulation of the Leibnizian cosmological argument. I will therefore use his treatment of PSR as a foil.

In several places, William Lane Craig has endorsed the following restricted version of PSR:

(PSRc) Every existing thing has an explanation for its existence, either in terms of the necessity of its own nature or in terms of an external cause.

But it seems to me that there is an ambiguity in Craig's notion of a thing having an explanation  "in terms of the necessity of its own nature". As Craig defines the notion in Reasonable Faith (p. 107 of the 3rd edition) such a being is one that exists of its own nature, and thus has no external cause. But this definition allows for at least two epistemically possible sorts of beings that could play such a role: metaphysically necessary beings and factually necessary beings. Metaphysically necessary beings are such that their inner natures cause them to exist in all possible worlds. By contrast, the necessity of factually necessary beings is world-indexed. Thus, a being must meet at least two conditions if they are factually necessary at a given possible world. First, their inner natures render them uncaused, beginningless, and existentially independent. And second, while there are possible worlds at which they fail to exist, they are de facto indestructible in the possible world at issue -- i.e., nothing else exists in that possible world that has what it takes to knock them out of existence.

It's important to note that both metaphysically necessary beings and factually necessary beings stand in contrast to beings whose existence is explained in terms of an external cause. Furthermore, both sorts of beings stand in contrast to brute facts -- i.e., beings whose existence have no explanation at all. Because of this, we should distinguish two versions of Craig's PSR to account for the distinct glosses:

The Metaphysical Necessity Version of PSR (PSRmn): Every existing thing has an explanation of its existence, either in terms of the metaphysical necessity of its own nature or in terms of an external cause.

The Factual Necessity Version of PSR (PSRfn): Every existing thing has an explanation of its existence, either in terms of the factual necessity of its own nature or in terms of an external cause.

Given these distinctions, it seems to me that one could accept a version of PSR -- viz., PSRfn -- even if one did not accept the existence of metaphysically necessary beings. For it's epistemically possible that all contingent dependent beings are ultimately composed of factually necessary beings (i.e., contingent independent beings). So, for example, perhaps matter-energy is a factually necessary being. According to such a scenario, the contingent dependent beings (e.g., rocks, trees, planets, you and I, etc.) come into being when two or more factually necessary/contingent independent beings are combined, and the contingent dependent beings cease to exist when they decompose into their elements. However, the elements of which contingent dependent beings are composed (i.e., the factually necessary beings) can't pass away, for there is nothing around in the actual world that has what it takes to knock these uncaused, beginningless, existentially independent beings out of existence.

On this picture, then, we have an explanation for all contingent dependent beings in terms of contingent independent beings. Furthermore, we have an explanation of contingent independent beings in terms of the factual necessity of their own nature -- i.e., in terms of their eternality, existential independence, and de facto indestructibility. Is PSR violated in this scenario? It depends: PSRmn is, but PSRfn is not. It therefore seems to me that a naturalist can accept a version of PSR without thereby committing herself to the existence of a metaphysically necessary being.

The point I argued for above, if at all on track, is significant in itself. However, I think there is promise for the naturalist to go further than this and to argue that PSRfn is to be preferred to PSRmn on theoretical grounds. Here's a sketch of how this might be argued. First, PSRfn appears to at least match PSRmn in terms of explanatory scope, as both versions provide an explanatory terminus for the existence of contingent dependent beings.[1]   Second, PSRfn is more theoretically conservative and qualitatively parsimonious than PSRmn with respect to the ontology it entails.

Finally, beyond its theoretical virtues as an explanatory principle with respect to actual phenomena, PSRfn arguably does a better job of explaining our modal intuitions with respect to merely possible phenomena. So, for example, we seem to have little or no trouble imagining or conceiving of worlds in which God does not exist.  We also have no trouble imagining or conceiving of contingent independent beings (i.e., factually necessary beings). So if conceivability is at least prima facie evidence of metaphysical possibility, then the existence of contingent independent beings, and the non-existence of God, are both prima facie metaphysically possible. But if either is possible, PSRmn is false. Neither possibility poses a problem for PSRfn, however.

In this way, then, one might plausibly argue that PSRfn has more going for it than PSRmn. But if it does, then PSR is not only consistent with naturalism; it's more at home in a naturalistic universe than in a theistic one.
[1] One might reply that PSRmn has wider explanatory scope, as it can also explain the existence of contingent independent beings (i.e., factually necessary beings), if any such beings exist. However, this is mistaken, as the latter sorts of beings are beginningless, uncaused, and existentially independent essentially.


cadfan17 said…
Is there any explanation available of how a "being" could be a valid target for the adjective "necessary?"

I get how mathematical formula can be necessarily true. Its truth has to do with the relationships between ideas. As long as identity is preserved between possible worlds, the mathematical relationships between mathematical concepts should remain constant.

I don't get how a being could be necessary.
exapologist said…
Ah -- a true Humean!

Well, I suppose a proponent of Anselmian theism would say that a metaphysically necessary being is one that exists at all metaphysically possible worlds. Explaining exactly how a being could be necessary in this sense is no doubt a bit trickier. ;) In any case, I'm here granting at least the intelligibility of the notion of such a being. Of course some theists want to infer possibility from intelligibility, but I think that's implausible (we have no problem understanding reductios and per impossibile reasoning, for example).
cadfan17 said…
Right, I know what religious people mean these days when they say metaphysically necessary. And I get that you're granting it here for the sake of the discussion.

I just don't understand why its supposed to work as a concept. I get necessity as applied to math. There's an actual account of why that makes sense. I don't get necessity as applied to a being. It reads to me like "infinite justice" or "infinite mercy": the application of an adjective to a noun that can't structurally support it (try mapping the set of justice onto a subset of justice and see how far you get). I was wondering how theistic philosophers claim that it can.
Dr. Rizz said…
Dear EA,

This is an excellent post. I have a question. If one grants that there are possible worlds in which very powerful God(s) exists that can destroy any physical object in those same worlds,wouldn't this wreak some havoc with attempts to define factually necessary beings using possible world semantics?

For example, let's stipulate that the actual world contains a universe composed of energy that can not be created or destroyed in the actual world. Furthermore, let us stipulate that there is another possible world in which this same universe exists uncreated (to avoid Kripkean origin necessity problems), and there is also a being of either infinite or nearly infinite power in that world that has the ability to destroy the universe in that world. Wouldn't we want to say that the actual physical world is factually necessary even though it is metaphysically possible to destroy it?

I would prefer a definition of factual necessity that sticks to the facts, as it is unclear to me how other possible worlds should figure in to the definition of the broadly speaking causal necessity that is involved here.
Dr. Rizz said…
Just read your very relevant footnote (1). I don't know how I missed that. I want to revise my comments a bit. Consider the following counter-example to your "strongly de facto" notion of indestructibility. Assume there is a world W1 at which there exists a universe that can not be destroyed even though there exists a being at that world that is almost powerful enough to do the job. There is another possible world W2 close by that world in which the same deity that exists in W1 is powerful enough to destroy the universe. Wouldn't we still want to insist that the being in W1 is factually necessary at W1 because it can not fail to exist at W1.
exapologist said…
Hi Dr. Rizz,

Very nice. I'm inclined to agree with your criticism of my account of strong de facto indestructibility. It'll probably take me a while to flesh out formally what I'm after in the notion (although I should've known to stay away from attempting a conditional analysis of the notion, given the well-known problems with conditional analyses of dispositional properties generally), but in the meantime I'll just stick to some variant of weak de facto indestructibility.

Dr. Rizz said…

I am inclined to think that weak de facto indestructibility is the essence of factual necessity. I also think that factual necessity is a strictly world-bound property. In the main text you talk about indestructibility at all worlds in which an entity exists. Even if you have in mind weak de facto indestructibility at all worlds in which a factually necessary being exists problems will still arise. For example, if our universe is weakly indestructible in the actual world, there are possible worlds in which this same universe exists (albeit uncreated in order to avoid Kripke’s origin essentialism difficulties) that also contain a deity powerful enough to destroy it. Yet, even if this is the case we would still want to say that the actual universe is factually necessary in a broadly-speaking causal sense at the actual world.

Now that I have re-read your original post I think that there are two ways in which you suggest that factual necessity is not a world-bound property. There is the multiple world consideration associated with strong de facto indestructibility and one that concerns your requirement that a factually necessary being is indestructible at all possible worlds in which it exists. If you eliminate both of these requirements you still have a modest and yet very explanatory principle of sufficient reason.


Marc Belcastro said…

I’m having some difficulty discerning why the existence (or non-existence) of a factually necessary being (FNB) isn’t a brute fact. Take two possible worlds, W1 and W2. In W1, some FNB exists. In W2, the aforementioned FNB doesn’t exist, nor does any other FNB. With respect to W1, what explains the existence of the FNB, and, with respect to W2, what explains the non-existence of that particular FNB?

Perhaps I’m not fully appreciating your account, but it seems to me that your account of the nature of FNBs is conditional: if some FNB exists, then “their inner natures render them eternal, existentially independent, de facto indestructible beings at all possible worlds in which they do exist.” But this doesn’t appear to explain why a FNB exists in the first place – if it does, in fact, exist. If a FNB happens to exist in some world, then I think your account explains why the FNB doesn’t fail to exist. That is, your account explains why the FNB is permanent, enduring, and never destroyed, but it doesn’t explain why a being with such properties even exists. For, in the spirit of Leibniz, we might ask: why does an eternal, existentially independent, de facto indestructible being exist (rather than fail to exist) in W1?

Lastly, regarding the nature of a metaphysically necessary being (MNB), we have an explanation of why such a being is permanent and never destroyed, and we also have an explanation of why it even exists in the first place. So, I guess I’m suggesting that there’s a distinction between (i) explaining why X is permanent and never destroyed, and (ii) explaining X exists at all. Your account addresses (i), but it doesn’t appear to address (ii). Put differently, FNBs have an explanation pertinent to (i), but it seems to me that an altogether different explanation is needed in order to address (ii). By contrast, regarding MNBs, they have a single explanation for both (i) and (ii).

Forgive me if I’ve not articulated my concern very clearly.
exapologist said…
Dr. Rizz,

Great comments! I'm inclined to think you're right here as well. In fact, your previous comments started pushing me in the direction of the further point you raise here.


No I agree. I tried to bring out in my post that while PSRfn is satisfied by the scenario i sketched, PSRmn is not, and for the reason you nicely point out. Is this a point in favor of PSRmn vis-a-vis PSRfn? It depends. As I alluded to in the post, it seems to me that If our modal intuitions count as prima facie evidence for what's metaphysically possible, then there is pressure to say that no being -- not even a god -- exists in all possible worlds. But if so, then there's pressure to say that the question can have no answer (at least not in terms of a metaphysically necessary being), in which case there is pressure to say that it's not something that needs explaining (again, at least not in terms of such a being).

I don't have especially strong convictions in either direction on this issue, though. As I tried to indicate at the beginning of the post, I don't feel the force of the claim that no naturalistically acceptable entity could play the role of a necessary being. Here I'm bracketing that issue and just focusing on the issue of whether a naturalist could accept PSR without thereby being forced (or at least nudged) to accept a metaphysically necessary being into their ontology.

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