u/Aristologos

A common objection to libertarian free will is the demand for a mechanism. The assumption behind this challenge is that any genuine explanation of action must ultimately be cashed out in lower-level mechanistic terms, and that without such a mechanism the idea of agent causation is unintelligible or incoherent. The following argument targets that assumption directly.

• P1. A mechanism is an explanatory structure in which the behavior of a system is accounted for in terms of the organization of its parts, their states or activities, and the relations that connect them.
• P2. Within any mechanistic explanation, the explanatory force of a higher-level description depends on lower-level structures, such that each mechanistic account implicitly appeals to further underlying states, activities, or relations in order to be fully specified.
• P3. If every mechanistic explanation requires a further mechanistic explanation of the conditions that produce it, then either (a) the chain of explanation proceeds without end (infinite regress), or (b) the chain must terminate in some explanatory posit that is not itself further explained in mechanistic terms.
• P4. An infinite regress of mechanistic explanations does not amount to a complete explanation of why the system as a whole obtains, because each stage depends on a prior one, and no stage provides an independent account of the whole.
• P5. Therefore, any coherent mechanistic explanatory framework must terminate in at least one irreducible explanatory ground.
• P6. An explanatory terminus within a mechanistic framework does not constitute an explanatory failure, but marks the point at which the framework treats some element as basic for purposes of explanation, thereby delimiting the scope of mechanistic reduction rather than undermining explanatory coherence.
• P7. Libertarian free will, in its agent-causal form, holds that an agent can function as an irreducible source of a decision, such that the decision is not fully accounted for by prior mechanistic states, but instead originates from the agent as an explanatory terminus.
• P8. If mechanistic explanation is compatible with explanatory termination in general, then the presence of an irreducible agent-level terminus is not ruled out solely by appeal to the structure of mechanistic explanation itself.
• C. Therefore, libertarian free will is not ruled out as incoherent or unintelligible by the nature of mechanistic explanation alone.

The bottom line is that "but what's the mechanism?" is not, by itself, a decisive objection, because mechanistic explanation always requires some stopping point. If explanation in general permits such termini, then there is no principled reason in advance to rule out an agent functioning as one. To reject agent causation at that point, one would need an independent argument showing why agents cannot serve as explanatory termini in the first place. Simply insisting that there must always be a deeper mechanism already assumes the conclusion that agents are not such termini, which is where the reasoning becomes circular.

The "determined or random" argument usually goes like this: every decision is either determined by prior states or random. If it is determined, then you can't do otherwise. If it is random, then it isn't controlled by you. Either way, the conclusion is that free will doesn't survive.

This only works if "random" is being used in a very specific way. There are really two different things being bundled under that label. One is just the negation of determinism: events are not fully fixed by prior states and laws. The other is a much stronger idea, where events occur arbitrarily, like dice rolls with no structure or constraint.

The argument switches between these two distinct definitions of "randomness". In its initial presentation, randomness is taken to mean simply the negation of determinism. On this interpretation, the dichotomy appears exhaustive: either events are fully determined by prior states or they are not. That's just a structural distinction about causation. Nothing in that version automatically tells you what "not fully determined" has to look like. It could still involve constraints, regularities, partial dependence on prior mental states, reasons, or anything else that isn't strict determinism. It only marks a boundary: complete fixity on one side, openness on the other.

In the subsequent inference to the impossibility of free will, however, this notion of "non-determined" events is implicitly narrowed. The indeterministic option is reinterpreted as a form of metaphysical arbitrariness, in which decisions are unanchored from the agent and amount to brute, reasonless selection. The initial dichotomy is thereby transformed from a structural distinction between determinism and its absence into an evaluative opposition between determinism and pure luck. These are NOT equivalent, and they carry very different implications for agency.

Once that shift happens, free will gets evaluated against a target it was not originally contrasted with. It is first placed into a very broad category of "not determined", and then that category is silently reinterpreted as something far more restrictive, "pure luck". Unsurprisingly, free will doesn't fit that stronger interpretation, but that's only because the target has been changed.

The argument thus relies on an equivocation in the use of the word "randomness". The setup presents a broad dichotomy that seems exhaustive, but the critique of free will depends on a narrower and more loaded version of one side of that dichotomy. The persuasive force comes from treating those two versions as interchangeable when they aren't.

If you separate them cleanly, the conclusion that "free will is impossible" doesn't follow from the premises alone. At most, you get the claim that free will doesn't fit either strict determinism or a very specific notion of brute, unstructured randomness. Whether those exhaust all relevant accounts of agency is exactly what needs to be shown, not assumed through a shift in interpretation.