“An intelligent man is one who is capable of taking in knowledge until the natural limits of the species are reached.. A stupid man is one whose progress is arrested at some specific time and place before then. There thus appears in psychology - and the next instant in politics - the concept of the unteachable. Some men can learn almost indefinitely; their capacity goes on increasing until their bodies begin to wear out. Others stop in childhood, even in infancy. They reach, say , the mental age of ten or twelve, and then they develop no more. Physically, they become men, and sprout beards, political delusions, and the desire to propagate their kind. But mentally they remain on the level of schoolboys.”
Book:Notes On Democracy