Evidence and Leverage: Comment on Roush

Sherrilyn Roush’s Tracking Truth provides a sustained and ambitious development of the basic idea that knowledge is true belief that tracks the truth. In this essay, I provide a quick synopsis of Roush’s book and offer a substantive discussion of her analysis of scientific evidence. Roush argues that, for e to serve as evidence for h, it should be easier to determine the truth value of e than it is to determine the truth value of h, an ideal she refers to as ‘leverage’. She defends a detailed method by which the value of p(h/e) is computed without ‘direct’ information about p(h) but only using evidence about the value of p(e), from which the value of p(h) is derived. She presents an example of how to use her leverage method, which I argue involves a certain critical mistake. I show how the leveraging method can be used in a way that is sound—I conclude with a few remarks about the importance of distinguishing clearly between prior and posterior probabilities