From Greg Mankiw:
In other words, the plan would reduce the deficit if it were carried out as written, but there is good reason based on historical experience to be skeptical that it would be.
Let me try to put CBO’s point in a more familiar setting:
Your friend Joe, who says he want to lose weight, asks you for an extra slice of pie after dinner. Naturally, you are doubtful about the wisdom of the request.
“Ahem, Joe,” you whisper, “Aren’t there a lot of calories in that?”
“Yes,” he says, “but the pie is part of a larger plan. I am committed not only to eating that slice of pie but also to going to the gym every day for the next week and spending at least half a hour on the treadmill. That exercise will more than work off those extra calories.”
“But that’s what you said last week, when you asked for piece of cake. And you didn’t go.”
“Yes, I know” he replies ruefully, “but this time I really mean it….Can you please pass the pie?”