(CNN) 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.

Cillizza: Rod Rosenstein urged "caution" for people reading stories about the Trump White House that contain anonymous sourcing. Is that caution warranted? Why or why not?

: Caution is always advisable toward stories based on anonymous sources. Yes. Even the people who produce these stories would probably agree with that. But if Rod Rosenstein wanted to warn us away from a report he knew to be misleading, his statement fails . It suffers from the same vagueness of origin that casts doubt on the statements of confidential sources.

Which revelations is Rosenstein talking about? What is the predicate for his actions in releasing this statement? What is that phrase, "when they do not identify the country..." doing in there? He seems to want to say more, but something prevents him. Or maybe he wanted to say less, but someone forced him. Either way, his statement is opaque. Which is the whole problem with confidential sources. They are opaque.

Cillizza: Let's take a step back. What is the history of anonymous sources? Where did this all start?

Rosen: I don't think we know for sure. One of the oldest dynamics in politics — reaching back to the 18th century Parliaments — is when the loser in an internal conflict decides to "change the game" by going public with a dispute that had previously been kept in house. Since they still have to live in that house, these people tend to be anonymous. This method in political combat first became possible when there were printed journals reporting on Parliament, and coffee houses where public questions were being debated— assisted, of course, by the newspapers of the day, on sale in those establishments.

Roughly speaking, then, the origins of anonymous sourcing, the birth of public opinion as a live factor in politics, and the invention of political reporting all occur together, in the mid 1700s. And that dynamic I identified — loser in an internal dispute goes public, hoping that the reaction will change the outcome — continues unchanged to this day.

Cillizza: Is anonymous sourcing on the rise?

Rosen: I would be cautious about any statement like that. That would take a massive content study to determine with any reliability. Such studies are done, but they tend to lag 5-7 years behind the fact.

One thing we can point to is the rise of publications — Politico, Business Insider, Axios would be three — that in my view simply don't care how often they have to rely on blind sources. They have made the call that getting the inside dope is more important to their readers than any fussy concerns about transparency. They have decided not to worry about it. In fact, Henry Blodget, founder of Business Insider, has said , "We will grant anonymity to any source at any time for any reason." Pretty clear statement. The existence of such competitors obviously has an effect on the climate as a whole.

Cillizza: President Obama's White House was notoriously hard on leakers. Trump seems to have taken that to a new level. Accurate reading?

Rosen: It's : It's well established that Obama was more aggressive against leakers than any previous president. Judging by his public statements , Trump is even more willing to go to war, but I don't think we know what is going on behind the scenes. The f irst criminal case against a leaker began this month. It's plausible to suspect that a lot is happening on this front, but we simply don't know enough to say for sure.

Cillizza: Finish this sentence: "Anonymous sources are _________ in modern journalism."

Rosen: "Anonymous sources are withdrawals against the bank balance built up by more transparent practices in modern journalism."

News accounts that rely on confidential sources do not contain within themselves the information required for us to trust them. By definition we cannot "go to the source" because the source is hidden. If we extend our trust to such reports, we do so because of reputation: the reporter's reputation, or more often the news brand's.

Some acts of journalism are easier to trust than others. If I tell you what the data shows about test scores in different schools around your district, and I also link to the data so you can check for yourself, that is a fundamentally different act from ... "The special counsel overseeing the investigation into Russia's role in the 2016 election is interviewing senior intelligence officials as part of a widening probe that now includes an examination of whether President Trump attempted to obstruct justice, officials said." (My italics.)