To avoid potentially endless controversy and ensure smooth operation of the healthcare system, law and practice require a yes-or-no verdict about whether a person can make a particular decision.

A third assumption that pervades contemporary work on decisional capacity is that for practical purposes, a ruling on capacity must be all-or-nothing: either the patient in question has decisional capacity or she lacks it. Of course, in many contexts there is an obvious sense in which we can speak meaningfully of “degrees” of capacity, for we are indeed measuring abilities that fall along a spectrum. However, in practice we need a judgment of a bivalent type. This is because we need to know who