 The category of “radical cognitive disability” is stipulative. We do not start with the assumption that any specific human being falls into that category, or even that some human beings do. Eva Kittay (2005) has argued that there is no reason to assume that any human beings are radically cognitively disabled, in the sense we are using the term. Jeff McMahan (2009) has countered that the existence of such human beings is very likely: given the continuous nature of fetal and infant neurological development, it is very likely that some human beings are radically disabled because their development has ceased or been interrupted at points where they have not yet acquired morally relevant capacities. Even if McMahan’s “existence argument” for radical impairment is correct, however, it does not remove the daunting uncertainty of attributing radical cognitive impairment to any actual human being.