Who's Still Afraid of Determinism? (Part 3)

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(Part One, Part Two)

In the previous entry, we encountered Dennett and Taylor's two main arguments contra the incompatibilists. In this brief final entry, we consider the appendix to D & T's article in which they deal with Peter Van Inwagen's famous Consequence Argument.

The Consequence Argument is supposed to show that agents cannot have meaningful causal powers in a deterministic universe. The argument is of moderate complexity. It is outlined and illustrated in the following diagram (click to "embiggen"):

As you can see, the idea is that because determinism implies that events in the remote past are sufficient for events in the future, no one really has control over those future events.

D & T attack the argument on the grounds that it conflates causal necessity and causal sufficiency. As we saw the last time, D & T argue that counterfactual necessity is the most crucial criterion of causal power. Thus, they think that the term "power to cause" in premise (3) of the Consequence Argument should be thought of in the following terms:

A has the power to cause a iff for some sentence ψ describing an action of A and a world f close to actuality, ψ ∧ a holds in f and a → ψ in every world similar to f.

What this means is that within a cluster of possible worlds close to our own, there is an action ψ that is a necessary condition for a to occur. If this sounds confusing or imprecise, I suggest going back to part one and looking at the definitions of possibility and causation that were outlined there.

The problem for Van Inwagen is that once this definition of causal power is employed, premise (3) as a whole becomes unwarranted. Why so? Well, premise (3) in its original form is claiming that a → ψ applies in a cluster of nearby worlds, and that a → b applies in all possible worlds. If we could deduce from these two claims that b → ψ then premise (3) would be justified. But we can't do this because elementary logic tells us that a → ψ and a → b do not entail that b → ψ.

And so, with this simple combination of definitional judo and elementary logic, the Consequence Argument fails. I should say, before leaving D & T's article behind, that many doubt that Van Inwagen's argument can be defeated in such a simple manner (D & T acknolwedge this fact in a footnote). I personally prefer Gary Drescher's suggestion that a causal link between practical reason is not always required for meaningful action. Drescher's argument is presented in his book Good and Real and in effect defangs the conclusion of the Consequence Argument.