Margins and Errors

Timothy Williamson has argued that cases involving fallible measurement show that knowledge comes apart from justified true belief in ways quite distinct from the familiar ‘double luck’ cases. I start by describing some assumptions that are necessary to generate Williamson's conclusion, and arguing that these assumptions are well justified. I then argue that the existence of these cases poses problems for theorists who suppose that knowledge comes apart from justified true belief only in a well defined class of cases. I end with some general discussion of what we can know on the basis of imperfect measuring devices.