Behind closed doors, negotiators can float trial balloons and make tacit offers — deniably. They can say things like, “This isn’t an offer, mind you, but just hypothetically, what if I were to suggest we could accept a Medicare cut if you could accept a capital-gains tax increase?” If you show hypothetical interest in my hypothetical offer, I can go and try it out on my caucus and constituents. If you wave me off — well, no offer was ever made, so I’m not embarrassed.

Often, the only way to get something done is to have separate private and public truths. Behind closed doors, nothing is settled until everything is settled. Until the deal is done, everyone can pretend not to have decided anything. But the moment the conversation becomes public, plausible deniability ceases. Everyone knows I’ve made an offer. Angry interest groups, adversaries in the other party, and even purists in my own party start cutting attack ads and lining up challengers to prevent a deal and defeat me.

In diplomacy, having two faces is similarly indispensable. Until recently, the existence of the United States’ use of drones for targeted killing was classified — not because it was a secret (everyone knew about it, especially the targets) but because public acknowledgment would embarrass key allies. As long as we pretended not to tell, they pretended not to know.