One may conclude from the passage that a full understanding of the nature and the capacity of human intelligence _____ .

Scientists have long sought ways to define and measure
human intelligence. And while theories of intelligence
have grown more sophisticated since the 1800s when
some believed mental abilities were determined by the
size of a person's head, researchers still do not agree
about certain fundamental principles of human thought.
They, therefore, continue to debate such basic questions
as whether heredity or the environment is more important
in forming intelligence.