Content knowledge can certainly help, but it is neither a prerequisite, nor a sufficient condition. We all know of people without formal education whose fluid intellect far outsmarted other people’s resounding credentials. Conversely, it often takes a very long college career for people to devise or even simply accept absurd ideologies.
Could the etymology of the word be of any help? Going the Heideggerian route, one could say that intelligence refers to the ability to “read between the lines”: inter-lego, as the latin language indicates. Indeed, to be intelligent does mean being able to make subtle distinctions. No, the value of all final products is not a good measure of a country’s ability to produce wealth, as it leaves out all-important intermediate consumptions. No, a country’s ability to produce wealth is not a good measure of its human development if it doesn’t properly weight investment in health and education. No, the Human Development Index is not a good measure if it leaves out the distribution of income and doesn’t take into consideration sustainability…
But then, “intelligence” is also the exact opposite: the ability to com-prehend things, i.e., to grasp and connect apparently unrelated information--to “connect the dots”, as the Indo-European root “bind together” would suggest. After all, intelligere means “to perceive”, and perception is “the expression of the many in the one”, as Leibniz explained.
So, what are we left with? What does it mean to be intelligent if "intelligence" means different things that seem contradictory: to be knowledgeable, but not to the point of losing common sense; to make constant distinctions, but also to grasp the connection between completely different things?
In the end, to be intelligent probably means to be humble, starting with what we think "intelligence" means. Not only does it entail an openness to different types of intelligences: it requires an actual effort to diversify our approaches and constantly cultivate multiple ways of thinking. Social intelligence, emotional intelligence, artistic intelligence, logical intelligence, kinesthetic intelligence…
As Pascal explained, intelligence is the ability to understand everything, but in the sense of finding and seeking intelligence and opportunities for learning everywhere. Being both rigorous and compassionate: that, maybe, is what being “intelligent” really means.