Among the much-repeated words woven into the US debate on national health policy are “affordability” and “sustainability.” Indeed, this debate is not confined to the United States. Remarkably, no one knows what these words actually mean at the practical level. Is there any economist or other expert, for example, who could be sure what percentage of the gross domestic product (GDP) the United States can “afford” to spend on health care, or what level of spending on Medicare is “sustainable”?