What is ignorance?

This article offers an analysis of ignorance. After a couple of preliminary remarks, I endeavor to show that, contrary to what one might expect and to what nearly all philosophers assume, being ignorant is not equivalent to failing to know, at least not on one of the stronger senses of knowledge. Subsequently, I offer two definitions of ignorance and argue that one’s definition of ignorance crucially depends on one’s account of belief. Finally, I illustrate the relevance of my analysis by paying attention to four philosophical problems in which ignorance plays a crucial role.