No occasion to exert his understanding

From The Wealth of Nations, Book 5, Part III, Chapter II by Adam Smith. Almost an argument that we can only be as bright as our circumstances permit which jives to some degree with the hypothesis explaining the Flynn Effect (more complex environments make us brighter).

The man whose whole life is spent in performing a few simple operations has no occasion to exert his understanding, or to exercise his invention in finding out expedients for removing difficulties which never occur. He naturally loses, therefore, the habit of such exertion, and generally becomes as stupid and ignorant as it is possible for a human creature to become.