“Whatever else they do, governmental commissions give reasons. In the Anglo-American political tradition, royal commissions and their nonroyal counterparts regularly form by executive order during national crises. Like reports from the National Research Council, they broadcast consensus among authorities, and thus aim to still controversy. Composed of distinguished citizens whose reputations shield them from charges of partisanship and self-interest, commissions usually call witnesses and issue reports. But at the end, they offer their own considered collective judgment on the matter at hand— their reasons.”