The article Decision-Making Capacity (Stanford Encyclopedia of Philosophy) says that any successful theory of decisional capacity must turn out that most ordinary adults count as having capacity most of the time (inclusivity). This is understandable for practical reason (say, people won't vote for policy based on otherwise theories). But why would that be a necessary requirement for a theory? For me it feels like: because most people believe that the sun orbits the Earth, therefore any astronomical theories shouldn't contradict to that. Is that correct? And in general, would inclusivity and tolerance be constraint for any theory of ethics?
Below is the full explanation the article provides:
The inclusiveness constraint derives entirely from the needs of practice. No matter what theory of decisional capacity we develop, it must turn out that most ordinary adults count as having capacity most of the time (Buchanan & Brock 1989: 21; Appelbaum 1998). In other words, as a society we are morally committed to imposing minimal restraints on individual choice. Most people are free to make most choices in their lives for themselves, even including self-harming choices. It would therefore be intolerable if our account of decisional capacity in medicine were to differ significantly from that norm, especially given that the doctrine of informed consent arose in the context of a movement aiming to give greater decisional power to patients and research subjects. Thus, it is generally recognized that failure of a theory to be inclusive enough would be sufficient by itself for rejecting an account of decisional capacity.