Free Will, Agent Causation, & Metaphysical Naturalism: A consistent and plausible combination



It’s no longer uncommon for free will to be met with suspicion. This suspicion is even greater when it comes to libertarian free will, and overwhelming regarding agent causation. This belief is largely arrived at via the notion that agent causation or even free will in general is inconsistent with Metaphysical Naturalism. This attitude is mistaken. Here I propose to show that even an agent causal account of action is consistent with Naturalism, which implies that free will in general is. Finally, I’ll close by arguing that at least some people are justified in believing in free will.


Metaphysical Naturalism (MN) is a meta-philosophical position regarding the fundamental nature of Being, the world, etc. What it entails is largely debated, but I will be using two definitions that are generally accepted.

MN1: Everything that exists is natural. There are no supernatural entities or forces.

MN2: Reality is exhausted by space-time and its contents, or an ensemble of space-time manifolds.

MN1 is the most common version, but it’s largely uninformative because “natural” is left unaddressed. We’re merely left with picking out paradigmatic supernatural entities/forces such as ghosts, gods, magic, and the like, and asserting that nothing of the sort obtains. I prefer M2, but I think assuming the truth of either one of them is sufficient for what I hope to demonstrate.

Free Will

To understand why people assume agent causation is inconsistent with MN, we have to clarify what free will is. First, the will is the capacity to deliberate, make decisions, and translate those decisions into action1. I take the folk conception of free will to mean that persons are sometimes able to exercise their will such that they could have done otherwise. That is, at least some decisions aren’t necessitated by their nature and/or environment2

More clearly, an action is free only if it satisfies the following conditions:

Sourcehood: The agent is the actual source of ones action (e.g. no manipulation).

Intelligibility: The agent performs actions for reasons that are understood by the agent (e.g. a spontaneous jerk isn’t a free action).

Leeway: The agent is able to refrain from performing the action.

It’s often assumed that naturalism entails determinism, and that determinism is in conflict with the leeway condition, and by this very fact naturalism is in conflict with free will. But this entailment does not hold. There’s nothing about naturalism itself that implies that all causal relations are determinate (necessitated by the relevant antecedent conditions). All that’s required of causality on MN is that nature is causally continuous. Which means that there is only one metaphysical causal kind within the world (i.e. Dualism is false), and that there aren’t external non-natural causal forces affecting the natural world. For these would almost be by definition supernatural. Further, contemporary physics already admits indeterminism in at least six interpretations of quantum mechanics (three remain agnostic, and four are explicitly deterministic)3. So if one is going to reject free will in virtue of MN, it can’t be because of MN entailing determinism. One might object that indeterministic events don’t take place in higher-level settings, such as the firing of a neuron, so a naturalistic interpretation of human behavior will be deterministic. First, there’s nothing about naturalism in itself that requires this. Second, whether some events in the brain operate indeterministically is an empirical thesis that remains to be settled, and there are already models of how this might work4

Given what has been outlined above, we can make sense of an event causal libertarian account of free will fitting within MN. In these sorts of instances, one’s mental states cause one to act but in such a way that you could have done otherwise. That is, the features of yourself that cause the action wouldn’t necessitate the action. You could have refrained or performed an altogether different action. It’s also helpful to note that this model fits nicely with the reductive account of mind, where any token mental state is identical to a particular brain state. Most philosophers specializing in free will recognize event causal libertarianism as a possibility worth considering, even if they remain skeptical of its reality5.

Agent Causation & Substance Causation

This charitable tone tends to drop once agent causation is proposed. This is typically followed by accusations of anti-scientific and “spooky” metaphysics. This is primarily grounded in the assumption that agent causation implies substance dualism. They can’t imagine what this agent could be besides a disembodied mind that interacts with the body. I think the agent causal picture people have in mind is much like how Kant thought freedom of the will worked. Essentially, the physical world that we experience is fully deterministic. Everything runs like clockwork with the exception of human action. In addition to bodies, persons are also noumenal selves that transcend the empirical world, making sovereign unconstrained choices each time they deliberate and act. So on this picture, the world consists of two different sorts of causes, natural events and agents. Given this sort of description, it’s of little surprise that so few philosophers take agent causation seriously.

Before we contrast the previous description with how agent causation has been recently updated, it will be useful to offer a brief description of what event causation is supposed to be. Event causation essentially involves some complex state of affairs or process causing another. For example, a heart pumping causes the movement of blood or a brick being thrown causes the window’s shattering. Further, the way these events unfold are explained by whatever laws of nature happen to obtain, be they deterministic or probabilistic. Causation cashed out as event relations can either be understood as ontologically primitive or reducible to something more basic such as facts concerning the global spatiotemporal arrangement of fundamental natural properties or sequential regularity.

Timothy O’Conner offers two similar, but philosophically distinct analyses of causation which clearly sketch the relevant difference between event and agent causation6:

Event causal analysis: “The having of a power P by object O1 at time t produces effect E in object O2.”

Agent Causal analysis: “Object O1 produces effect E, doing so in virtue of having power P at time t.”

In the first case it is the “possessing a power”, an event, which is the cause of the effect; in the second it is the object. What’s of crucial importance here is that the agent causal analysis isn’t actually just one of agent causation, but is of the more general theory of substance causation. Substance causation is just the theory that substances or objects are what cause effects. So on this account, it’s not the throwing of the brick that causes the window to shatter; properly speaking, it’s the brick. Now this might sound absurd; how could the throwing of the brick not be a cause of the windows breaking? The absurdity drops once we consider the thrower. Really, the thrower and the brick jointly cause the windows shattering, where the throwing is a manifestation of a power possessed by the thrower. Powers theory is crucial to any plausible theory of substance causation. It’s not merely the object in itself that causes the effect, but the nature of the object that is constituted by the powers it possesses.

Most of the mysteriousness of agent causation disappears once we understand it as a species of substance causation. So take any ordinary substance, a rock, an electron, a water molecule, etc; any time any substance causes an effect on another substance, we have an instance of substance causation. What distinguishes agent causation from ordinary instances of substance causation is that there is an intention behind it. This entails that agent causation is fairly common place within the animal kingdom, which itself is good reason to believe that agent causation is consistent with naturalism.

A robust defense of substance causation is beyond the scope of this paper, but I can briefly sketch some reasons for accepting it. One is the numerous problems with alternative theories of causation. The constant conjunction or sequential regularity theory is currently one of the most popular and has been since Hume proposed it. On this account, for x to cause y is just for it to be the case that every time x occurs, y occurs. So on this view there is no intrinsic or necessary connection between the fire and the smoke that follows; this is just the way the universe happens to unfold. A contentious assumption on this theory is that all instances of causality are temporarily ordered. But we can make sense of non-temporal causation such as two cards propping each other up or a ball making an impression on a pillow that it’s been resting on for eternity(i.e. there was no prior time where ball was not affecting the pillow).

The other popular account reduces causation to counterfactual dependence, which is something like this,

1) If A had not occurred, B would not have occurred.

2) If A had occurred, B would have occurred.

3) A and B both occurred. “ (Scholastic Metaphysics, pg. 60).

So the throwing of the brick causes the window breaking because if you remove the throwing of the brick then the breaking would not have happened. One problem with counterfactual dependence is the infinite number acts of omission that are involved in any causal sequence. So my successfully walking across the street was dependent on not being crushed by an elephant, not being transported, the earth not blowing up, etc. Another issue that’s applicable to both theories is that both of them seem to get the dependence relation wrong. It’s because of causation that there is constant conjunction and counter factual dependence. They are symptomatic of causation.

Next, here is a simple argument in favor of substance causation7:

1. Some actual substances possess causal powers.
2. If a substance possesses a causal power, then it is efficacious.
3. If a substance is efficacious, then it can be a cause.
4. Some actual substances’ causal powers are manifested.
5. Therefore, some actual substances are causes.

The only premise I can imagine being rejected is (1). On the face of it, this might sound absurd; as if it means that nothing has the power to do anything. Though really the individual who rejects causal powers would have alternative explanations for why things do what they do. A not uncommon answer is that we only need appeal to the laws of nature to understand and explain how events unfold. This is problematic. On one hand, if you take the laws of nature just to be descriptions of regularity, then the laws themselves don’t do any explanatory work. On the other hand, if you take the laws of nature to be something that dictates and enforces the activity of things from the outside, then you’ve committed yourself to a form of platonism, where naturalism must be rejected. Finally, you can take the laws themselves to be the causal implications of the intrinsic natures that the substances possess, and in that case we’re back to powers theory.

Metaphysical Irreducibility

One might object to my earlier claim that agent causation is fairly common place because in reality there are no agents, merely matter in motion or atoms in the void.This is where the possible reducibility of macro-level objects becomes an issue. So a largely reductionist metaphysics will hold that much of what we consider ordinary objects are nothing over and above their parts. So what they are is wholly reducible to a set of fundamental constituents and relations. Another way to think of about this is that if we were to take an inventory of everything that really exists, much of what we take to exist would turn out to not. At its most extreme, the reductionist thesis holds that there’s nothing over above quarks, bosons, or whatever a complete theoretical physics takes to be fundamental. Ordinary objects will be described as simples (indivisible physical objects) arranged in a particular way. So to be a cat is just to be simples arranged cat-wise.

If one were both a reductionist and a substance causation theorist, then one could rightfully reject agent causation because there would be no agents in the relevant sense. In order for agent causation to obtain, the agent has to be a unique substance that’s not merely the sum of its parts. If agent causation were true, then agents would be irreducible substances whose persistence conditions are picked out by their higher-level causal powers(e.g. Purposiveness, narrativity, & self-reflection). That is, we are unique irreducible substances because we possess capacities that aren’t exemplified by our constituents. The constituents have come together in the right way; they are not merely a collection of them. A unique form is exemplified that puts constraints on the activity of its lower-level constituents. Which is an example of top-down causation if anything is. On reductionist substance causation, the lower level substances do all of the causal work.

A possible strategy for motivating a non-reductionist account mirrors the demystifying of agent-causation. That is, if irreducible objects aren’t special cases that are essentially restricted to persons, then there’s less reason to be suspicious of irreducibility in general. This does not mean that I think that all ordinary objects are irreducible substances. I take objects of artifice to be clearly reducible to their chemical constituents. So houses, cars, computers, tools, etc are reducible to their constituent parts. Edward Feser offers a clear description of the distinction I have in mindviii,

The basic idea is that a natural object is one whose characteristic behavior – the ways in which it manifests either stability or changes of various sorts – derives from something intrinsic to it. A nonnatural object is one which does not have such an intrinsic principle of its characteristic behavior; only the natural objects out of which it is made have such a principle. We can illustrate the distinction with a simple example. A liana vine – the kind of vine Tarzan likes to swing on – is a natural object. A hammock that Tarzan might construct from living liana vines is a kind of artifact, and not a natural object. The parts of the liana vine have an inherent tendency to function together to allow the liana to exhibit the growth patterns it does, to take in water and nutrients, and so forth. By contrast, the parts of the hammock – the liana vines themselves – have no inherent tendency to function together as a hammock. Rather, they must be arranged by Tarzan to do so, and left to their own devices – that is to say, without pruning, occasional rearrangement, and the like they will tend to grow the way they otherwise would have had Tarzan not interfered with them, including in ways that will impede their performance as a hammock. Their natural tendency is to be liana-like and not hammock-like; the hammock-like function they perform after Tarzan ties them together is extrinsic or imposed from outside, while the liana-like functions are intrinsic to them” (Scholastic Metaphysics, pg. 182)8

I don’t commit myself to the idea that all natural particulars are irreducible or simple (without parts) or that only objects of human construction are reducible. For example, a rock made of limestone would reduce to a collection calcium carbonate, that may or may not have an irreducible intrinsic nature. The correct account of reduction/non-reduction relation is a severely under-explored issue in metaphysics. The hope here is merely that this example is useful in communicating an idea of what an irreducible relation/substance is supposed to be.

Final Arguments

Before summing up the arguments, it’ll be useful to explain what sort of advantage an agent causal account of freedom has over an event causal one. It stems from what’s called the “disappearing agent” objection to event causal libertarianism. The idea is that on the event causal analysis the agent-involving events (the particular mental states, preferences, reasons, etc) that non-deterministically cause the decision don’t actually settle which option is selected. The leeway condition is satisfied in that we could roll back the event and you could have otherwise but you, yourself don’t actually choose it. Your agent-involving states merely constrain which options are possible for you. Where it goes from there is a matter of luck. This can be thought of as claiming that an event causal view doesn’t satisfy the sourcehood condition for free will. The events, which do the work, merely flow through you, but you don’t really settle which option occurs. Agent causal theories have the advantage of saying that you certainly do play an explanatory role.

With this work behind us, we can abridge the essential story into a few brief arguments.

1. Substance Causation is consistent Naturalism.
2. The metaphysical irreducibility of certain substances (persons among them) is consistent with Naturalism.
3. If (1 & 2), then agent-causation is consistent with Naturalism.
4. Therefore, Agent Causation is consistent with Naturalism.

I think 1 and 2 are fairly straightforward in that nothing about my description of them implied that they transcend space and time, and 3 isn’t much more than the definition of agent causation.


1. The leeway condition is consistent with Naturalism (i.e. Nothing about naturalism implies that all causation is deterministic or that all causally relevant neural sequences are deterministic).
2. The sourcehood condition is consistent with Naturalism (since the most demanding form of satisfying it (agent causation) is consistent with Naturalism).
3. The intelligibility condition is consistent with Naturalism (I can’t say much more than I’d be completely puzzled if someone denied this, beyond maybe saying that all of our reasons for action are post hoc confabulations).
4. If (1,2 & 3), then Free Will is consistent with Naturalism (A priori true).
5. Therefore, Free Will is consistent with Naturalism.


1. Substance causation is a plausible theory of causation.
2. The irreducibility of certain biological substances is not implausible.
3. Indeterminism is plausible.
4. If (1,2, & 3), then free will is plausible.
5. We’re justified in holding independently plausible positions if they cohere with our background beliefs*.
6. Therefore, at least some people are justified in believing in free will.

Plausible: A position is plausible just in case it is coherent, contains sophisticated arguments or evidence in favor of it (ones that are aware of and address the relevant issues and objections that might undermine it) and is void of any obvious insurmountable objections.

*Epistemic axiom: We’re justified in believing what seems to be true unless we have sufficient reason to think it’s false.

*Phenomenological claim: Some of our decisions seem to be free, to at least some of us.

Without question, this is the weakest of the arguments I’ve offered. Plausibility is context dependent, which means many will find this unconvincing. Some of the most obvious candidates are committed reductionists, scientismists, eliminativists, determinists, and event causal theorists. Though this is not my target audience. My hope is that fence sitters, or anyone who’s just generally skeptical yet open to free will and agent causation might be persuaded to take the position seriously. No one should be moved to believe in free will merely based on what I’ve offered here, but it might be sufficient to motivate some to re-assess their position.


1 Franklin, Christopher Evan, Agent-Causation, Explanation, and Akrasia: A Reply to Levy’s Hard Luck, Criminal Law and Philosophy 9:4, (2015): 753-770.

2 I’m assuming incompatibilism, but even a compatabilist might find the agent causal argument interesting and useful. See

Markosian, Ned 1999: ‘A Compatibilist Version of the Theory of Agent Causation’. Pacific Philosophical Quarterly, 80, pp. 257–77.

——2012: ‘Agent Causation as the Solution to all the Compatibilist’s Problems’. Philosophical Studies, 157, pp. 383–98.


4 Peter Ulric Tse, Neural Basis of Free Will: Criterial Causation, MIT Press, 2013, 456pp.
-Christopher Evan Franklin, The Scientific Plausibility of Libertarianism’, Free Will and Moral Responsibility, eds. Ishtiyaque Haji and Justin Caouette. Newcastle upon Tyne: Cambridge Scholars Publishing (2013): 123-141.
-Indeterminism in Neurobiology. Marcel Weber. Philosophy of Science, Vol. 72, No. 5, Proceedings of the 2004 Biennial Meeting of The Philosophy of Science AssociationPart I: Contributed PapersEdited by Miriam Solomon (December 2005), pp. 663-674

5 For an excellent treatment of event causal libertarianism see:

Balaguer, Mark. Free Will as an Open Scientific Problem. MIT Press, 2010

-Balaguer, Mark. A Coherent, Naturalistic, and Plausible Formulation of Libertarian Free Will. Noûs, Vol. 38, No.3 (Sep., 2004), pp. 379-406

6 O’Conner, Timothy. “Free Will and Metaphysics,” in David Palmer, ed., in Libertarian Free Will (ed. D. Palmer, Oxford), 2014

7 Whittle, A. (2016). A Defence of Substance Causation. Journal of the American Philosophical Association , 2(1), 1-20. DOI: 10.1017/apa.2016.1

8 Edward Feser, Scholastic Metaphysics: A Contemporary Introduction, Editiones Scholasticae, 2014, 302pp.