# A Causal Model for the Principle of Alternative Possibilities

The Principle of Alternative Possibilities says that S is morally responsible for P iff it is possible that S did not perform P.

In his 1969 book Alternate Possibilities and Moral Responsibility Harry Frankfurt introduced a much discusssed counterexample to the Principle. His counterexample runs as follows:

Suppose that Smith really wants Jones to perform some action. He sets out to ensure that Jones will perform this action. Smith resolves to wait until Jones makes up his mind about what to do before he acts. Once Jones resolves what to do, Smith will step in to ensure that Jones performs the desired action. As Smith watches, Jones deliberates and then elects to do the action Smith desired all along!

Frankfurt concludes from this example that Jones is certainly morally responsible for his action, even though it's true that Jones could not have done otherwise.

In this post I'd like to offer an explanation for the force of the counterexample based on its causal model.

## The Model

An oversimplified first pass at the causal model of the situation Jones is in looks like this:

deliberation --> action

In Frankfurt's original case, Smith knows the value that deliberation will take. If it takes an certain value (say 0), then Jones will act in the way undesired by Smith. Smith observes the value that Jones takes in deliberation and then Smith will step in to ensure that action takes the desired value, in case deliberation takes the value 0. If deliberation takes the value 1, then Smith does nothing. This is effectively a plan to intervene on the variable action.

While it's true that Jones could not have done otherwise, this fact isn't due to a feature of the causal model itself (I'm just going to stipulate this, although you might think if a model like this was veridical and deterministic there would be bigger threats to Jones's freedom than Smith) but due to the way the intervention is being applied to the model. The performance of the intervention is conditional on the values for deliberation. It seems to me that the force of Frankfurt's example is that Jones is morally responsible for his action (despite being unable to do otherwise) because of the value that deliberation takes.

However, suppose that Smith intervened on deliberation instead. My reaction to this intervention is that Jones isn't responsible at all. Part of this is the way intervention gets formally modeled. There's no way for Smith to observe that deliberation has taken value 0 and then intervene "before" that value is transmitted to action. The intervention and subsequent downstream effects are simultaneous. The only way to get Frankfurt's case is to suppose another exogeneous variable exists:

brain activity --> deliberation --> action

Now Smith can observe the brain activity variable for its value and intervene on deliberation to prevent the value from getting passed downstream. I think the main problem at stake with this approach is that it begins to look less and less plausible that Frankfurt's counterexample is going to do the work of undermining the Principle. If the causal model for Jones' behavior looks like the one here, then it's already false that Jones could have done otherwise.

A few lingering thoughts:

1. It would be interesting to see how the model would differ in a stochastic case.
2. Is the best way to understand Smith's action as an intervention?
3. Could Smith be intervening not on a variable in the model but on the model's structure?