Abstract
This is a paper about time travel and what it teaches us about freedom. I argue that cases of time travel bring out an important difference between two ways of thinking about “the past”—either in terms of time itself, or in terms of causation. This ambiguity naturally transfers over to our talk about things like fixity, determinism, and incompatibilism. Moreover, certain cases of time travel suggest that our freedom is not constrained by the temporal past per se, but by our own causal histories. This, in turn, raises a number of interesting questions about the formulation of incompatibilism and the broader debate over freedom.
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Notes
Many of the arguments in this paper were inspired by thinking through related problems that arise in connection with human freedom and divine foreknowledge. See Wasserman (2022).
More carefully, we will take determinism to be the thesis that any physically possible worlds with the same past also have the same future. Of course, there are many ways of formulating determinism, and not all of them make reference to the past (see, e.g., Earman, 1986: 13). However, our focus will be on the “past-to-future” version of the view, since that is the one at issue in debates over freedom.
For simplicity, I will assume that that there is a single “version” of Marty present at each time. Also, in contrast to the movie, I will assume that time has a non-branching structure. Finally, for the sake of simplicity, I will assume an “eternalist” picture of time on which the past, present, and future all exist and are equally real. For more on the relation between theories of time and the possibility of time travel, see Wasserman (2017: Chapter 2).
See Wasserman (2017: 6–8) for more details on refining Lewis’s notion of personal time. (Note too that the concept of personal time can be applied to non-persons such as time machines.)
I focus on psychological continuity for illustrative purposes only. Other views of personal identity will refer to causal processes involving one’s body, memory, etc.
For simplicity, we can assume that causation is transitive, so that Marty’s causal past includes everything that is a cause of a cause of a cause… of any event in his life. For further clarifications and refinements, see Sect. 3.
I assume that the incompatibilist will take this to be a necessary truth (and that the same thing will be true for the fixity of the past). Moreover, while incompatibilism is typically formulated in terms of conditionals, it is natural to read it as an explanatory thesis: actions in a deterministic world are not free precisely because those actions are determined by the past and the laws (which we have no control over). We will return to this point in Sect. 3.
As Isaac Wilhelm pointed out to me, it is unclear how to think about the laws that figure into personal determinism since they do not take world-states as inputs. Moreover, as a reviewer for Philosophical Studies pointed out to me, the causal past and the laws (however those are understood) will not be enough to determine a unique personal future since our futures are influenced by elements outside of our past lightcones. At best, your causal past and the laws determine your present state (including your present actions). Fortunately, that should be enough (given the rest of the incompatibilist’s argument) to show that no one is free at any time to do anything other than what they do at that time.
We will return to this issue in Sect. 3. Note that PI is an agent-relative principle—it says that an agent’s freedom depends (in part) on facts about their own personal history, where those facts will differ from person to person. For example, PI tells us that Biff is free to kill Marty in 1985 only if that action is compatible with Biff’s causal history at that time (as well the laws); Marty, on the other hand, is free to kill Biff only if doing so would be compatible with Marty’s causal history at that time (a causal history that differs, in important respects, from Biff’s).
For an extended explanation and defense of this reasoning, see [blinded].
For a survey of ways that Marty could consistently kill his ancestor, see Carroll (2016).
We will consider a non-obvious reason in Sect. 3.
One reviewer questioned the veracity of this claim on the grounds that it would give Marty the power to “make happen things that did not happen”. However, there is an important ambiguity in this way of describing things. My claim is that there is some event, e, such that: e did not happen, but Marty had the power to make it happen. This should be contrasted with the claim that Marty had the power to make it the case that: there is some event e that both did and did not happen. Marty obviously lacks this second ability, but that is no reason to deny him the first.
Ginet (1990).
Mele (1995).
Clarke (1993).
As several readers have reminded me, one could try to block this objection by (i) understanding “facts about the external past” in EI as referring to “hard” facts about the external past and (ii) claiming that the fact that the rock was not kicked in 1885 is not a hard fact about the external past in 1955 (given the presence of Marty’s time machine). However, it is difficult to provide an interpretation of “hard facts” that would deliver this result without turning fixity into an uninteresting truism. For example, if “hard” was taken to mean “something that no one can change”, then the fixity of the past would amount to the claim that no one can change any of the facts about the past that no one can change.
In fact, Rea argues for the stronger claim that time travelers undermine the freedom of everyone around them.
We can also define x’s “causal past” at t as the sum of all events satisfying (D1). See Rea (2015: 271). If we assume that causation is transitive, we can replace this condition with a requirement for causation itself.
Cf. Fischer (1994: 78).
See Vihvelin (1996).
For simplicity, I will take actions to include “refrainings”, such as Marty’s refraining from kicking the rock.
(FCP) may require even further refinement in light of Law and Wasserman (2022).
This conclusion fits with a broader shift in the literature, where the threat of determinism is now often framed in terms of prevention, control, or sourcehood, rather than non-causal notions like logical entailment. See, for example, Sartorio (2014), who also mentions the relevance of time travel to this issue (see, especially, pages 260-2).
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Acknowledgements
“I am grateful to Western Washington University for professional leave that allowed me to work on this paper. I am also grateful for feedback from Andrew Law, Christian Lee, Frances Howard-Snyder, Dan Howard-Snyder, Hud Hudson, Dee Payton, Neal Tognazzini, Dennis Whitcom, Isaac Wilhelm, and anonymous reviewers from several journals, including Erkenntnis.”
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Wasserman, R. Time Travel, Freedom, and Incompatibilism. Erkenn (2023). https://doi.org/10.1007/s10670-023-00661-y
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DOI: https://doi.org/10.1007/s10670-023-00661-y