Bayesianism and self-doubt

Synthese https://doi.org/10.1007/s11229-020-02879-7 Bayesianism and self-doubt Darren Bradley1 Received: 13 February 2020 / Accepted: 16 September 2020 © The Author(s) 2020 Abstract How should we respond to evidence when our evidence indicates that we are rationally impaired? I will defend a novel answer based on the analogy between self-doubt and memory loss. To believe that one is now impaired and previously was not is to believe that one’s epistemic position has deteriorated. Memory loss is also a form of epistemic deterioration. I argue that agents who suffer from epistemic deterioration should return to the priors they had at an earlier time. I develop this argument regarding memory loss then extend it to cases of self-doubt. Keywords Self-doubt · Forgetting · Bayesianism 1 Introduction How should we respond to evidence when our evidence indicates that we are rationally impaired? Christensen (2010) and Schoenfield (2018) have argued that there is a tension between conditionalization and the belief that you are rationally impaired. I offer a novel theory of how we should respond when we believe we are rationally impaired. I will argue that an extension of conditionalization applies in cases where the agent loses information, and then argue that cases of self-doubt can be assimilated to cases of losing information. Section 2 explains a case of believed rational impairment and Sect. 3 describes Christensen’s and Schoenfield’s positions. Section 4 shows how agents can respond to memory loss, then argues that a similar response is called for when self-doubting evidence is acquired. Section 5 applies the account to a series of cases that build up to the original case of believed rational impairment. Section 6 argues that this strategy can be used to guide agents by taking a step back and focussing on the synchronic constraints needed for both memory loss and self-doubt. Section 7 discusses objections. Section 8 concludes. B Darren Bradley 1 Philosophy Department, Woodhouse Lane, Leeds University, Leeds LS2 9JT, UK 123 Synthese 2 Drug case Where there is a source of information, there is room for doubt about whether it is trustworthy. We’ll focus on two sources of information—memories and reason. We can doubt our memories and we can doubt that we are reasoning correctly. Let’s start with Christensen’s example1 which is most directly a case of self-doubt about reasoning: Drug Suppose first that I’m a rational scientist investigating some phenomenon experimentally, and suppose that, were I to get evidence E, it would give me excellent reason for confidence in G (say, because E is highly unexpected, and G would be a terrific explanation for E). And suppose it’s Sunday, and I’ll get the results of my experiment when I get to the lab Monday morning. In this case, it seems that these things may well be true of me: 1. I’m not highly confident that G is true. 2. I am highly confident that if I will learn E tomorrow, G is true. 3. If I get to the lab and learn that E is true, I should become highly confident that G is true. So the confidence in G that I should adopt Monday at the lab, if I do learn E, lines up with the confidence I have Sunday that G is true on the supposition that I will learn E on Monday. Now instead of considering just the possible experimental outcome E, let’s consider a more complex bit of evidence I could acquire Monday morning. I could learn not only E, but D: that a powerful explanation-assessment-disrupting drug is slipped into my breakfast coffee on Monday. Here, it seems that a gap opens up between the two things that lined up nicely before. First consider how confident I should be that if I will learn (E&D) tomorrow, G is true. My being drugged tomorrow has no bearing on the actual evidential/explanatory connection between E and G, and no independent relevance to G. So it seems that, today, I should think that, if E is true, G is very likely true, whether or not I get drugged tomorrow morning. Thus: 4. I am highly confident that if I will learn (E&D) tomorrow, G is true. But if I actually do learn (E & D) tomorrow, will it in fact be rational for me to become highly confident in G? It seems not–after all, if I learn D tomorrow, it will not be rational for me to trust my assessments of explanatory support. And this is true whether or not I’m actually affected by the drug. So it seems, at least at first blush, that: 5. If I go to the lab and learn (E&D), I should not become highly confident that G is true. So it seems that the higher order evidence about my being drugged produces a mismatch between my current confidence that G is true on the supposition that I will learn certain facts, and the confidence in G that I should adopt if I actually learn those facts. 1 See Schoenfield (2018, p. 690) for a similar example. 123 Synthese (Christensen 2010, pp. 199–200 Notation altered.) It will be useful to abstract away from the details: G  Hypothesis E  Evidence which supports G G and E are eternal i.e. non-self-locating propositions. We can add to the story that you are confident throughout in the non-self-locating proposition that a powerful explanation-assessment-disrupting drug is slipped into your breakfast coffee on Monday. And we need to add the self-locating proposition that it is now Monday: D-  It is now Monday Given the assumption that you are confident throughout that a powerful explanationassessment-disrupting drug is slipped into your breakfast coffee on Monday, we can make the significance of D- more explicit: D  It is now Monday, a time at which I am impaired On Sunday you are unimpaired and disbelieve D. On Monday you believe D and so do not believe you can infer G from E. We also need to add to the story that on Monday you don’t trust your memories. Otherwise, if you did trust your memory on Monday, you could recall that on Sunday you believed that G was likely given E, and could just defer to your earlier credences. So we must ‘add to the description of the [possible impairment] that it also distorts one’s memories of one’s previous credences’ (Christensen 2010, p. 200). Let me make five clarifications. First, you trust your reasoning ability and memories otherwise. Second, you are never in fact impaired, you just believe you are (or will be). Third, you always know what time it is. Fourth, you move from being sure that you are not impaired to believing that you are. (My analysis is not intended to apply to a case where you think that you might have been impaired all along. In such a case there would be no need for memory loss so I think that conditionalization could be applied, although I won’t argue for this here (see fn. 10.) These assumptions will be in play for the whole paper. An assumption we will drop in Sect. 6 is that a single unique credence is obligatory given any particular set of evidence; call this Uniqueness.2 Uniqueness A single unique credence is obligatory given any particular set of evidence 3 Christ (...truncated)