In this interview with Joel Schneider in Scientific America he explains how intelligence is a ‘folk concept’ (something close to an essentially contested concept), an interesting explanation:

In describing intelligence as a folk concept, I do not mean that it is a primitive idea in need of an upgrade. Many folk concepts are incredibly nuanced and sophisticated. They do not need to be translated into formal scientific concepts any more than folk songs need to be rewritten as operas. Of course, just as folk melodies have been used in operas, folk concepts and formal scientific concepts can inform one another—but they do not always need to. For this reason I want to dispense with the trope that there is something fishy about the subdiscipline of intelligence research because groups of psychologists persistently disagree on the definition of intelligence. They do not need to agree, and we should not expect them to. If they did happen to agree, the particular definition they agreed upon would be an arbitrary choice and would be non-binding on any other psychologist (or anyone else). That is the nature of folk concepts; their meanings are determined flexibly, conveniently, and collectively by the folk who use them—and folk can change their minds.

Further, to say that something is a folk concept does not mean that it is not real or that it is not important; many of the words we use to describe people— polite, cool, greedy, dignified, athletic, and so forth—refer to folk concepts that most of us consider to be very real and quite important. Intelligence, too, is very real, and quite important! In fact, it is important by definition—we use the word to describe people who are able to acquire useful knowledge, and who can solve consequential problems using some combination of logic, intuition, creativity, experience, and wisdom.

See what I just did there? I tried to define intelligence with a bunch of terms that are just as vague as the thing I am trying to define. Of course, terms like useful knowledge andconsequential problems are abstractions that take on specific meanings only in specific cultural contexts. If you and I have a shared understanding of all those vague terms, though, we understand each other. If we are of the same folk, our folk concepts convey useful information.
(read more)