The naturalism-versus-theism dialectic features numerous arguments and counter-arguments stemming from all areas of philosophy. One prominent battleground focuses on the concept of simplicity – specifically, trying to determine which worldview can account for the various features of our reality, while, at the same time, utilizing the fewest metaphysical (epistemological, semantical, etc.) concepts in order to do so. The idea being, all things being equal, the simplest solution is generally the best solution (a principle often termed ‘Ockham's Razor’).

Specific to the natural-versus-theism debate, Graham Oppy remarks,

As Oppy proceeds to explain, while naturalism is committed to: (1) an account of the natural universe and (2) the natural universe is all that exists; theism, in contrast, is committed to (1) plus (2*) there exists a theistic realm, and (3) the natural realm and the theistic realm are all that exists (Ibid). Thus, all things being equal, via Ockham's Razor, one should favor naturalism over theism.

Let's grant that theoretical simplicity is determined, at least largely, by metaphysical simplicity.Footnote ¹ That is, in comparing hypothesis¹ over hypothesis,² all things equal, whichever hypothesis endorses a more modest ontology should be preferred. And let's even grant that postulating a multiverse in order to accommodate fine-tuning is ontologically simpler than a hypothesis that postulates God and one universe at the level of ultimate reality. While this second assumption doesn't compart with our intuitions about simplicity, we assume it for the sake of argument.

We argue still, that the already committedFootnote ² Classical Theist shouldn't find Oppy's argument from simplicity compelling. The proponent of the type of Classical Theism that we have in mind is going to endorse the potency and act distinction, she is going to endorse a participatory metaphysics, and she's going to think that existence comes in degrees. The familiar potency-act distinction distinguishes between the way things are and the way things could potentially be. For example, the coffee on Mike's desk is warm, but it has the potential to be cold (say, if Mike let's it sit on his desk overnight), or the potential to be boiling hot (perhaps Mike rewarms it in the microwave for too long). Potentialities (coldness, hotness, sharpness, dullness, roundness, flatness, etc.) are real features of objects (coffee, pencils, balls, etc.) that have yet to be actualized. Classical Theists think that behind all change is ultimately being that is simply pure act.

By a participatory metaphysics, we have in mind the view that there is a distinction between existence or esse, and essence.Footnote ³ (For example, the Classical Theist thinks that Pikachu has an essence, but yet, lacks existence.) Existence is what is fundamental to ultimate reality. Unless one's essence is to exist, one's essence has to come together with existence and participate in it. Essences which come to exist in this way, can be said to have derivative existence. These essences can be said to exist, but they exist to some lesser degree. Having said all of this, we can now see why the Classical Theist (at least of the stripe mentioned) shouldn't be convinced by Oppy's argument.

Traditionally, ultimate reality is what is of concern in the debate between theism and naturalism. By ‘ultimate reality’, we simply mean the correct account of the nature of everything or of all being that exists at the highest degreed level of reality. Generally, for naturalists, all objects of our experience, such as humans, houses, and stars, are part of ultimate reality. The Naturalist is then committed to the existence of composites (act and potency compounds) as features making up ultimate reality. The Classical Theist, however, denies this. These things exist but not as features that make up ultimate reality. Objects exist in a derived and less fundamental sense. As to what exists in ultimate reality, theism simply postulates that there is pure act. Theism is then simpler.

Now, let's say that the naturalist gives up on debating about what one posits at the level of ultimate reality. Instead, she will settle for considering what is simply fundamental to her worldview. Recently, several theists have made a similar move. Perhaps she will argue that the existence of quarks or perhaps an initial singularity is what is fundamental to her view. There's no need to worry about anything else she says. Nonetheless, quarks or an initial singularity would still be reducible to act and potency compounds. In contrast, what is fundamental to the Classical Theist's view is simply pure act. Either way you slice it then, the Classical Theist won't see the need to abandon ship.

OK, the naturalist says. Let's not determine simplicity by way of what's fundamental to the respective hypotheses, but rather, let's determine which hypothesis is simpler by an analysis of all things one postulates, even if those things don't makeup ultimate reality. Perhaps this is how we should have understood Oppy this whole time. The naturalist postulates composites while the theist postulates both pure act and composites. Naturalism is therefore simpler.

Not so fast. This might move the naturalist, but this scenario won't get off the ground for the Classical Theist who endorses participatory metaphysics. Recall that the Classical Theist thinks that in order for some essence to exist, it must participate in existence itself. If the theist equates pure existence with pure act (as they normally do), then she will take it that the naturalist is already committed to both the existence of pure act and the existence composites. Of course, the naturalist might not be convinced that pure act just is God, as there is the whole problem with the second-stage of arguments for God's existence. We needn't get into all of this here. Nonetheless, the question that is being debated is whether an already committed Classical Theist should be convinced by Oppy's argument. If the Classical Theist assumes participatory metaphysics, then she needn't concede of a possibility where one can have existent things without existence itself. Participatory metaphysics is part of the Classical Theist's background knowledge, k. The Classical Theist likely thinks alternative metaphysics of existence are incoherent and, for completely independent reasons, implausible. To be moved by Oppy's argument from simplicity, what is needed are additional reasons to reject k. Only upon developing these reasons, will the Classical Theist find Oppy's simplicity concerns more credible.

Now, we want to clarify, that our response here shouldn't be seen as an argument for Classical Theism. We recognize that one could argue that if we assume the sort of participatory metaphysics discussed here, then we are already assuming the existence of God and the simplicity concerns become irrelevant. Now, we don't think this is necessarily the case due to there being a second-stage or gap problem from arguing from pure act to God. But as mentioned earlier, this needn't concern us here. What's important is to clarify that we are simply arguing that Classical Theists who accept the aforementioned theses won't find Oppy's Simplicity argument compelling because of the metaphysics they assume. In fact, the Classical Theist might be turned off (perhaps rightly) to the idea that we can even in principle prove or disprove God's existence by way of looking at Classical Theism as a scientific hypothesis. Since God is, after all, not a thing or an object of the universe, why think that scientific methodology plays a role in establishing what grounds the realm of existent things? Nonetheless, even if we concede that such methodology is appropriate, we have shown why Oppy's argument from simplicity shouldn't move the Classical Theist of the stripe that we have imagined.