In "Are we free to break the laws?", David Lewis denies the principle:
- If I can do A, and were I to do A, B would be the case, then I can make B happen.
Let us say that I could have rendered a proposition false in the weak sense iff I was able to do something such that, if I did it, the proposition would have been falsified (though not necessarily by my act, or by any event caused by my act). And let us say that I could have rendered a proposition false in the strong sense iff I was able to do something such that, if I did it, the proposition would have been falsified either by my act itself or by some event caused by my act.The Lewis's point is that the strong sense is what captures our intuition that we can't make law-violations happen.
But now consider this case. Somebody offers me a hundred dollars for an electrical impulse going from my brain to my toes. I quickly realize: this is easy—all I need to do is to wiggle my toes, since when I wiggle my toes, a nerve impulse will have been sent from the brain to the toes.
Now we can extend Lewis's definition to the ability to render a proposition true by replacing "false" with "true" and "falsfied" with "made true". I obviously have the ability to send the electrical signal in the weak sense: I can wiggle my toes, and were I to wiggle my toes, the signal would be sent.
Question: Do I also have the ability to send the electrical signal in the strong sense? At least prima facie, no. For my wiggling of my toes does not cause nerve impulses to go from the brain to the toes. Nor does the wiggling of my toes itself make it true that an electrical signal is sent along the pathway.
But notice that the sense in which I can make it be so that an electrical signal goes from my brain to my toes is pretty robust—I can in fact make the sending of the signal a means towards further ends of mine. So if I cannot make the signal go from brain to toes in the strong sense, the strong sense is too strong—it doesn't capture our intuitions about making something happen.
Perhaps, though, we can take "the wiggling of my toes" to include everything involved in that intentional action, starting from the intending, including the sending of electrical impulses to the toes, and culminating with the motion of the toes. If so, the wiggling of my toes under these circumstances (not under circumstances where I have some non-electrical prosthesis replacing the nerves going to the toe muscles) does seem to make it true that a signal goes from the brain to the toes.
But now we have a problem. For if we take action in this expansive sense, then it will include various neuron firings as well. (If it doesn't, we can change the example: someone offers you a hundred dollars for when such-and-such a neuron fires, and you figure out that it fires when you raise your arm, so you raise your arm.) But, on Lewis's scenario, in the counterfactual world where you violate the (actual world's) laws, it's pretty likely that one of these neuron firings will be the violation of the laws. Hence it is the action—or at least a part of it (and here Lewis must allow a part-to-whole inference, or else the wiggling of the toes case comes back)—that is a violation of the laws.
But Lewis still has one more move. If L is the laws, a miraculous firing of a neuron does not by itself falsify L. For the firing of the neuron is consistent with L. What is not consistent with L is that the firing should have occured in prior conditions C. To falsify the laws, one would need to not only bring about the firing of the neuron, but also the prior conditions C, since the violation of the laws logically requires the two. But if Lewis insists on this point, then he has failed to capture what we mean by "break the laws" in his account. For suppose Samuel is an Uruguayan citizen by birth, and perhaps contrary to fact let us suppose that it is not possible to renounce Uruguayan citizenship. Suppose, further, Samuel fails to vote in the elections. I understand it is illegal to fail to vote in Uruguay. We would say that Samuel's failure to vote violates the laws. However, Samuel's failure to vote is not logically sufficient for a violation of the relevant law. What is logically required for there to be a violation of the law is that (a) one fail to vote, and (b) one be an Uruguayan citizen. But Samuel did not bring about (b). So to break a positive law it is not required that one do something that is or causes something logically sufficient for a violation of the laws. When in circumstances C the laws require A, to fail to do A is to break the laws. By the same token, when in circumstances C the laws of nature require that a neuron not fire, the firing of the neuron in C is a breaking of the laws.
In sum: One can send an electrical signal from brain to toes in a robust sense, a sense in which we cannot break the laws. But an account of "can" that allows this will also allow that one can fire neurons, say when raising an arm. And an account of "can" that allows this will also allow one to say that if determinism is true and yet one can act otherwise, then one can break the laws. And so, I suspect, Lewis is wrong.