Survival in the everyday world (in both social and practical functioning) depends on one's ability to recognise and avoid going down the worst possible path, especially when doing so places one at risk of death, injury, or social disaster. Most people possess "common sense" (the ability to recognise obvious risk) but some people lack that ability and thus are at high risk of engaging in "foolish" (i.e., risk-unaware) action. People who have a cognitive impairment are much less able to recognise and avoid risk, and this is what causes them to be seen as needing protection and support. In this paper, we argue that the answer to the question "What is intellectual disability (ID)?" is more likely to come from the question "What is unintelligent behavior?" than "What is intelligence?" The answer which comes from such a question is that "ID is a common sense deficit disorder characterised by unawareness of obvious social and practical risk." Several implications of this answer are explored for the field of intellectual disability. These implications are explored primarily for adults who may have ID, given that the inspiration for this paper came from the way existing ID definitions are applied or misapplied in the US adult criminal justice system. (Contains 6 figures.)