So far we've discussed common sense definitions of stupidity. It tends to be described as a lack of something - either cognitive horsepower ('intelligence'), or knowledge, or thinking. This seems inadequate. Defining it only as an absence of brainpower fails to account for what I'm calling fish-out-of-water stupidity. People with powerful brains who have acquired a great deal of knowledge in one domain, and who are therefore regarded as exceptionally smart, tend to assume they will have exceptionally smart thoughts in every field of knowledge they wander into. They take their own accumulated knowledge for granted and believe that the facility it gives them in their field is merely a function of their all-round brilliance.