Most people don't think they "could" cure Alzheimers by snapping their fingers, and so they don't feel terrible about failing to do this. By contrast, people who fail to resist overeating, or who fail to stop playing Civilization at a reasonable hour, feel strongly that they "could| Minding our way
You probably don't feel guilty for failing to snap your fingers in just such a way as to produce a cure for Alzheimer's disease. Yet, many people do feel guilty for failing to work until they drop every single day (which is a psychological impossibility). They feel guilty for failing| Minding our way