On Thursday, Obama met behind closed doors at the White House with tech company executives and privacy groups to discuss his administration's system of surveillance. The context was radically different from the last conversation he participated in on the subject, his chat with Jay Leno on The Tonight Show. But both conversations reinforce the ongoing problem with that surveillance: informed, public debate on the subject appears to be impossible.

The Thursday meeting was the administration's second this week. Politico reported on its attendees: Tim Cook of Apple, Randall Stephenson of AT&T, Vint Cerf of both Google and internet history fame, and groups including Center for Democracy and Technology, which has advocated for more openness on surveillance issues. The meeting was off-the-record, and no attendees spoke with Politico about what was said.

It was the second-such meeting this week, following one on Tuesday with a similar assortment of organizations, as reported by The Hill. In attendance at that one were representatives from much of the rest of the PRISM phalanx — Facebook, Yahoo, Microsoft — and from advocacy groups like the Electronic Privacy Information Center and the New America Foundation. Obama didn't attend the Tuesday meeting, but its guidelines were similar: no talking about it.

That stipulation for Thursday's meeting elicited a perhaps-predictable critique, as exemplified by The Guardian's Glenn Greenwald.

Obama did have a public conversation about the issues earlier this week, however — in that Leno appearance. And what did we get? Responses that were vague or misleading.

Which was also predictable, and not only because Obama has a vested interest in holding the conversation on his own terms. The vast, vast majority of those watching Leno have not been paying close attention to the NSA surveillance developments. They have probably heard of PRISM; they have probably not heard of XKeyscore. Obama's assurances that he meant to defend the nation and their personal privacy was superficial in part because there was not much depth he could explore.

Nor could Leno — or, if the White House meetings had been public, the privacy groups — have effectively challenged Obama on his answers. The president holds two distinct advantages in that debate: he necessarily knows more about what's going on, and there is an obvious power imbalance. Jay Leno is not a journalist. He's an entertainer. He's not interested in pressing Obama on detailed privacy issues. The privacy groups might want to be more forceful — and may well have been! — but doing so in front of cameras to an angry president is a significant demand for the head of a small non-profit.

(We'll note that the meetings almost certainly weren't only discussions about privacy. One likely additional topic for the tech companies present: the economic toll of compliance with the NSA's demands.)

At 3 p.m. this afternoon, the president will hold his first press conference since April 30. At it, the power balance is lessened. These are journalists used to confronting authority figures (in most cases), used to batting away contentious responses. And its where Obama will be bolstered or hampered by the second advantage above: there's some stuff he can't talk about. That can be a crutch — he can use it, for example, to defer questions about the programs' efficacy — but it can also be a liability. The president defers to his national security staff for their assessments of what should and shouldn't be public. Even challenging them to be more forthright (however sincerely) hasn't resulted in many new releases of information. The only recently declassified documents, those released by the director of national intelligence last week, we mostly released to embarrass Congress after the House nearly voted to defund phone metadata collection.

An open, public, informed conversation on surveillance has been the president's stated goal since shortly after the Edward Snowden leaks began. How authentic that desire is isn't clear. But how possible it is isn't either.