According to contextualism, the truth-conditions of knowledge attributions depend on features of the attributor's context. Contextualists take their view to be supported by cases in which the intuitive correctness of knowledge attributions depends on the attributor's context. Williamson offers a complex invariantist account of such cases which appeals to two elements, psychological bias and a failure of luminosity. He provides independent reasons for thinking that contextualist cases are characterized by psychological bias and a failure of luminosiy, and argues that some of our intuitions about the cases are explained by the former factor and some by the latter. I argue that psychological bias is the more fundamental of these elements. I show how, by itself, psychological bias can explain all the intuitions concerning contextualist cases. Further, it gives the best account of why contextualist cases are characterized by a failure of luminosity.