“Replaying Life’s Tape – No Miracles Required” — Extinct

Why alter? Why alter? Why alter? I mentioned miracles. The connection involves causation and determinism. Sensitivity of outcomes to initial conditions—which is at least part of the replay thought experiment—is in keeping with a prominent notion of causation as “counterfactual difference-making.” To say that antecedent event A1 caused outcome O1 is to say that, had A1 not occurred—had A2 occurred instead—O1 would not have resulted. The occurrence of A1 vs. A2 makes a difference. Where sensitivity to initial conditions goes further is to suggest that the occurrence of A1 vs. a slightly different A2 makes a big difference. But that’s not the important thing for now.

The important thing is to see how differently the criteria for counterfactual difference-making can be satisfied. Proponents of counterfactual notions of causation are understandably concerned to juxtapose what actually happens or happened with counterfactual situations that are relevantly similar; realistic if not real. To attribute the extinction of dinosaurs to an asteroid impact is to say, among many other things, that had the asteroid been called back at the very last second by the extraterrestrials who sent it, then the dinos would have lasted much longer. But that counterfactual is too unrealistic and is of little help when it comes to making sense of what actually happened. There’s a tradition of juxtaposing what happened with counterfactuals that not only take for granted the actual laws of nature but everything else that has happened in the real world up to and except for the putative causal event. Now, for a determinist, not only is it not the case, but it could not possibly be the case that the events of this world transpire exactly as they have, governed by our laws of nature, up to the event in question, at which point something else happens instead. But not to worry! The counterfactual world is not our world; it’s an alternative “possible world” like ours in all the above respects up to and except for the putative causal event. However, for determinists, the problem does not go away so easily, because events could no more play out in this way in the alternative deterministic world than in our deterministic world. Thus, proponents of this approach attribute to the alternative world what they dare not attribute to ours, namely “miracles” (yes)—“minor miracles” to be sure, but miracles nonetheless. There is a price to pay for determinism! Indeterminism also has its costs, to be sure, but the indeterminist has no problem with a counterfactual scenario in which events transpire in a specified way (according to stochastic laws of nature) up to a point where one of two or more alternative events could happen next. One needn’t resort to miracles happening in alternative worlds for appropriate counterfactual situations.

Doesn’t the determinist’s pickle sound like the predicament faced by proponents of the “altered” replay experiment, i.e., having to invoke miraculous or otherwise fishy alterations of the events to which the tape is rewound, so that the replay begins from a different starting point? The source of the problem may be the same. Determinism has been a major motivation for proponents of sensitivity to initial conditions. Sensitivity makes sense of the practical unpredictability of so many phenomena, but without abandoning determinism. Paraphrasing Edward Lorenz, the present determines the future, its just that the approximate present does not approximately determine the future.

Proponents of the “altered” version may think they’re doing Gould a favor by not attributing to him the kind of indeterminism that the “identical” version seems to embrace. But I don’t think he would have appreciated the generosity. Consider his last thoughts on related issues in The Structure of Evolutionary Theory. In the epilog to the final chapter he bemoaned the pervasive conception of scientific understanding that acknowledges the importance of initial conditions, together with laws of nature, but does not regard “the resolution of such details [the initial conditions] as essential or causal components of the explanation itself” (Gould 2002, 1332–1333). It’s not immediately clear what he meant by “the resolution” of the initial conditions, but I think he was saying that it matters to our understanding of the outcome how the initial conditions came about and especially whether they were matters of chance.

The line just quoted is followed by a parenthetical diatribe about how his undergraduates typically responded to the idea of real chance by parroting Laplace, insisting that the appearance of chance is just a matter of ignorance (this does seem like an undergraduate obsession), and moreover that “if science works at all, [it must] be truly deterministic” (Gould 2002, 1333). To which he responded,

Natural historians have too often been apologetic, but most emphatically should not be in supporting a plurality of legitimately scientific modes, including a narrative or historical style that explicitly links the explanation of outcomes not only to spatiotemporally invariant laws of nature, but also, if not primarily, to the specific contingencies [happenstance] of antecedent states [initial conditions], which, if constituted differently, could not have generated the observed result. (Gould 2002, 1333; my italics)

“The specific contingencies of the antecedent states” are not addressed by sensitivity to initial conditions. The question of their contingency is not only ignored, but the evidence is effaced by the “altered” version of the replay experiment. Strangely effaced. And at the cost of realistic counterfactual conditions for understanding what actually transpired.

There’s no need to rewind, alter in some miraculous or otherwise sketchy way, and then play. Just rewind and play. And enjoy. But watch out!


(* If you can’t get enough historical contingency, the original post contains an interesting back-and-forth in the comments section between John, Adrian, Derek, Will Bausman, and David Sepkoski. Also, here is John giving a talk on some similar issues back in 2014.)


Blount, Z.D., Borland, C.Z., and Lenski, R.E. 2008. Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli. Proceedings of the National Academy of Sciences, U.S.A. 105:7899–7906. doi: 10.1073/pnas.0803151105.

Gould, S.J. 1989. Wonderful Life: Contingency and the Nature of History. New York: W.W. Norton and Co.

Gould, S.J. 2002. The Structure of Evolutionary Theory. Cambridge (MA): The Belknap Press.

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