Abstract
In recent years, anthropic reasoning has been used to justify a number of controversial skeptical hypotheses (both scientific and philosophical). In this paper, we consider two prominent examples, viz. Bostrom’s ‘Simulation Argument’ and the problem of ‘Boltzmann Brains’ in big bang cosmology. We argue that these cases call into question the assumption, central to Bayesian confirmation theory, that the relation of evidential confirmation is universally symmetric. We go on to argue that the fact that these arguments appear to contradict this fundamental assumption should not be taken as an immediate refutation, but should rather be seen as indicative of the peculiar role that the relevant hypotheses play in their respective epistemic frameworks.
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Notes
For ‘simulation hypothesis’.
i.e. ‘It is not an essential property of consciousness that it is implemented on carbon-based biological neural networks inside a cranium: silicon based-processors inside a computer could in principle do the trick as well.’ (Bostrom 2003: p. 244)
These assumptions actually do very little work in what follows, and we could do without them. However, they serve to simplify the analysis significantly, and they don’t seem unreasonable in the context of the current dialectic.
To be clear, I take premise 2 to be evidence for SH in the sense that conditioning on premise 2 should raise the probability of SH.
To be clear, it is not important to our arguments that E should represent the entire relevant body of cosmological evidence. It is sufficient for our purposes that E be any piece of observational cosmological evidence confirming the \(\Lambda CDM\) model.
For example, the baryon acoustic oscillation feature and the polarisation of the cosmic microwave background.
\(\Lambda \) is also referred to as the ‘cosmological constant’ or ‘dark energy’.
To clarify, of course Bayesian confirmation theory does not itself rule out the consideration of BB. It simply precludes the possibility of a-symmetric EHP’s. But since there appear to be good reasons for claiming that BB is part of an a-symmetric EHP, advocates of Bayesian confirmation theory may be tempted to disregard the hypothesis as absurd.
The Boltzmann brains example satisfies these three conditions.
Here, one might be tempted to reply that Bayesianism is an extremely well supported and powerful epistemological framework. Thus, since the notion of a-symmetric confirmation is inconsistent within a Bayesian setting, the sheer weight of pragmatic and theoretical considerations on the side of Bayesian confirmation theory should be sufficient for us to dispel the idea. However, this kind of reasoning is at best overly conservative and at worst downright dogmatic. Of course, I don’t deny the manifold virtues of Bayesian epistemology, but the fact that Bayesianism is well supported and useful does not constitute a principled justification for ignoring those aspects of scientific reasoning that are incompatible with Bayesian principles like the universal symmetry of confirmation. The literature is replete with examples of cases where the Bayesian is unable to give a satisfactory account of manifestly rational forms of inductive reasoning (see e.g. Norton 2010a, b). For example, it is well known that the principle of indifference, which states that a rational agent who is indifferent over several possible outcomes should not assign any outcome a higher degree of belief than any other, leads immediately to inconsistency and paradox when formulated in a Bayesian setting (see e.g. Norton 2010a, b , Rinard (2013) , Van Fraassen (1989)). It would seem ad-hoc and dogmatic to simply reject the principle of indifference because it is inconsistent with classical Bayesianism. And indeed, several authors have suggested amending the standard Bayesian framework in order to resolve the paradoxes of indifference (see e.g. Joyce (2005) , Weatherson (2007) , Norton 2010a, b). This is a more fruitful and principled response. The mere fact that Bayesianism has achieved many successes does not mean that we should ignore its failures.
A similar analysis could be given for SH.
Many thanks to John Norton for pointing this out to me with and providing the following example.
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Acknowledgements
This work was generously supported by the Ludwig Maximilian University Center for Advanced Studies. I’d also like to thank John Norton and three anonymous referees for their helpful comments on earlier versions of the paper.
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Eva, B. A-symmetric confirmation and anthropic skepticism. Synthese 196, 399–412 (2019). https://doi.org/10.1007/s11229-017-1486-9
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DOI: https://doi.org/10.1007/s11229-017-1486-9