Abstract: Liberalism welcomes diversity in substantive ideas of the good but not in the process whereby these ideas are formed. Ideas of the good acquire weight on the presumption that each is a person's own, formed independently. But people differ in their capacities to conceptualize. Some, appropriately characterized as cerebral, are proficient in and profoundly involved with conceptualizing. Others, labeled cognitively disabled, range from individuals with mild limitations to those so unable to express themselves that we cannot be sure whether their behavior is mediated by concepts at all. Constricted cognitive capacities have been thought to prevent participation in the prescribed process for forming personalized ideas of the good. So liberal theory, when formulating principles and practices of justice, often disregards cognitively disabled peoples' perspectives. We put aside metaphysically driven notions about personhood and show how interpersonal processes of “prosthetic” thinking (different from surrogacy) can satisfy liberalism's standards, positioning cognitively disabled individuals as fully participating subjects of justice.