Dr. Truthlove or: How I Learned to Stop Worrying and Love Bayesian Probabilities *

Dr. Truthlove loves believing things that are true, and hates believing things that are false. She has just written an extensively researched book, and she believes every claim in the body of the book. However, she is also aware of the history of other books on the same subject, and knows that every single one of them has turned out to contain some false claims, despite the best efforts of their authors. Thus, one of the claims she makes, in the preface of the book, is to the effect that the body of this book too, like all the others, surely contains at least one false claim. She believes that too. She notices a problem. At least one of her beliefs is false. Either some claim from the body of the book (all of which she believes) is false, or else the claim from the preface (which she also believes) is. So she knows that she’s doing something that she hates — believing a false claim. At the same time, she notices a benefit. At least one of her beliefs is true! Either the claim from the preface is true, or all of the claims in the body of the book are true. So she is doing something that she loves — believing a true claim. But none of this answers the overall question. Is she doing what she ought to do? There is something apparently uneasy about her situation, but she can’t be sure whether it’s good or bad.