On an ordinary day, Henry Aaron, senior fellow in economic studies at the Brookings Institution in Washington, comes across as the quintessential policy wonk: knowledgeable, thoughtful, measured, perhaps even a tad boring. With his rumpled suits, snowy hair, and rosy jowls, the genial septuagenarian brings to mind one's favorite uncle--assuming that uncle had spent the past 40 years exploring tax policy, health care financing, and the intricacies of sprawling entitlement programs.