What anonymous sources cost journalism

Deputy Attorney General Rod Rosenstein put out a very odd statement late Thursday night. He attacked anonymous sources and insisted that any story using them is rightly viewed extremely skeptically. I reached out to Jay Rosen, a professor of journalism at New York University and the author of Press Think, a blog about media and politics, for some perspective on the history of anonymous sourcing and what it means for the future of journalism. Our conversation, conducted via email and lightly edited for flow, is below.