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I would like to thank my commentators for their generally supportive and astute comments. In this response I would like to clarify the differences that I perceive between my position and theirs. We have little to no disagreement about the neural processes that underlie decision, but regarding how to interpret those facts we do have substantial disagreement, and it is to these disagreements that I turn. Shadlen and I do not differ in our views about the importance of noise in decision making, but we differ greatly in our interpretation of what noise is and the metaphysical consequences of it. I think some of the confusion here can be allayed by paying attention to the distinction I made in my essay between apparent and metaphysical indeterminism. By eliding that distinction, I think Shadlen has erred in interpreting me, and in interpreting the relevance of my position for theories about free will. To review, apparent indeterminism refers to dynamics that have unexplained variance from the perspective of an onlooker, where the onlooker could be the scientist, or even a neuron that is receiving signals from a population of other neurons. Different activity in response to the (at least apparently) same conditions suggests apparent indeterminism. Crucially, however, these onlookers, whether scientists or neurons, do not have access to the entire system and cannot tell whether apparently identical conditions really are identical. Metaphysical indeterminism, the indeterminism that matters to libertarian arguments for free will, requires more than apparent indeterminism: It requires probabilistic events that do not follow solely from prior conditions and the causal laws. Instead of speaking in terms of apparent and metaphysical indeterminism, Shadlen talks about noise. Noise, I think we both agree, is non-task-relevant signals that cannot be dissociated by the observer from task-relevant signals. I do not agree with Shadlen, however, that “if the variation were caused by some irrelevant feature of the environment or the agent’s internal state, then the 3.3 Response to Commentators Adina L. Roskies 152 Adina L. Roskies decision mechanism should know to discount this variation.” Perhaps it would “know” to discount if the system were optimal, but it is not (see Beck, Ma, Pitkow, Latham, & Pouget, 2012). That leaves open what the metaphysical status of that noise is. Noise could be the reflection of metaphysically indeterministic processes, but it could equally as well be a reflection of deterministic processes, even processes that are a task-relevant signal for some other observer. As I explain in my piece, the libertarian requires metaphysical indeterminism, so merely pointing out the importance of noise does not per se do anything for libertarian positions. Thus, I think, Shadlen is wrong to say that neurobiology supports indeterminism; certainly he is wrong to say that that is my message. Shadlen and I fundamentally disagree about the nature of the noise generated by the brain. Shadlen says, “We do not live in a universe in which there is such thing as an infinitely precise definition of state.” I believe he means to say that we do not live in a universe in which there are infinitely precise states. His argument is that determinism is physical fiction, that no real system could be in a precise enough state for it to always evolve in the same way. I do not see any real argument for this claim although it may have some relation to the kind of point that Nancy Cartwright makes in How the Laws of Physics Lie (Cartwright, 1983). However apt her argument may be about the laws of our current physics and our ability to describe the states of the world, I do not see how those arguments apply to the realm of possibility—namely, to the claim that determinism could not, as a matter of physical necessity, possibly be true. Moreover, I believe quantum theory indicates that there are such precise states (defined by quanta), not that it shows the opposite. Regardless , neither I nor Shadlen is especially qualified to argue about fundamentals of physical theory, so let’s turn to the implications for brain science. Shadlen and I both countenance the reality of chaotic behavior in the brain, and we both believe that such behavior is one source of neural noise. Shadlen clearly believes that neural noise is metaphysically indeterministic since everything is. I, on the other hand, am agnostic about whether neural noise is metaphysically indeterministic or metaphysically deterministic. For just as chaotic dynamics can result from either indeterminism or from the...

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