Intelligence is the sense of knowledge balanced with the sense of uncertainty in that knowledge.
Its increase is found by support of active curiosity, exploration, and a willingness to admit that, for all one’s knowing, there is no such thing as “knowing”.
It is, thus, the paradoxical state of both/neither and the continuous, conscious choice to maintain the balance of it rather than slip into statis and stagnation by allowing one or the other permanent ascendancy.
“It is impossible for a man to learn what he thinks he already knows.” – Epictetus