Coherence and Reliability: Studies in Bayesian Epistemology

Abstract: In this thesis the connection between coherence and reliability is investigated. The question may be phrased as follows: does the fact that a set of testimonies is coherent imply that the witnesses who have reported these testimonies are reliable? The same question may also be expressed in terms of beliefs: does the fact that a set of beliefs is coherent imply that the beliefs were reliably produced? Traditionally, coherence theorists have thought that coherence is connected to truth, but in this dissertation the thesis is that it is rather connected to reliability. The investigation proceeds within a probabilistic framework, the so-called witness scenario, where a number of partially reliable witnesses give independent reports. Hence it is primarily coherent sets of testimonies that are discussed, but coherence of beliefs is taken to function similarly, and thus the results acquired using the witness scenario are, it is argued, relevant for the traditional discussion which mostly concerns coherence of beliefs. Using this scenario a notion of reliability-conduciveness, similar to the much-discussed notion of truth-conducivenes, is formally defined. This definition says, roughly, that a measure of coherence C is reliability-conducive if and only if the more coherent a set of testimonies is, as measured by C, the higher is the probability that a witness who has given one of these testimonies is reliable. In Papers I and II it is tested whether a number of coherence measures proposed in the literature are reliability-conducive in various salient scenarios. It is shown that the only coherence measure that is reliability-conducive in all of those scenarios is the so-called Shogenji measure. This observation is then used to argue that the Shogenji measure is a fruitful explication of the notion of coherence. However, in Paper III it is shown that in the most general scenario neither the Shogenji measure nor any other measure of coherence is reliability-conducive. In Paper IV, the use of coherence reasoning in court is investigated. It is shown that we often use the notion of coherence in judicial reasoning, for example to find out whether certain witnesses are reliable or not. Using the results from Papers I and II, as well as additional formal results that strengthen the connection between (Shogenji) coherence and reliability, it is argued that this line of reasoning is valid. Also, it is argued in the introductory chapter that these findings show that coherence is indeed closely connected to reliability, even though it is not generally reliability-conducive, and that this connection is much closer than that between coherence and truth. In Paper V, coherent sets of so-called higher order testimonies (and higher order beliefs) are examined, where a higher order testimony is defined as a testimony concerning some other witness’s reliability (higher order beliefs are defined analogously). Such sets have hitherto not been examined within formal coherence theory. It is shown that coherence must be defined in a non-standard way to account for all our intuitions regarding sets involving higher order testimonies. It is then argued that this shows that in order to correctly estimate the degree of coherence of a set of beliefs, the believer must have a firm grasp of her own reliability; a fact that in turn is taken to be problematic for the coherence theory of justification.