Ideas are like the wind and the air. We cannot see an idea, but we can see its effect on things. In the same way, a breeze by itself is not very much, maybe only enough to bend a small twig, but over the centuries it can shape a mountain. Air is the element that rules over ideas and abstract thought. We fight wars over ideas: a form of government, the possibility of buying and selling goods. We argue constantly about intangible, elusive things such as principles, morals, laws. An idea, like the wind cannot be contained, it cannot be forced into any particular shape, it always reforms itself and can never be grasped in one's hand.
Air is a natural ally of fire, it fans and feeds it. And anyone can see that Fire cannot survive without air. To continue further with the metaphor, an idea (air) is useless without action (fire): nothing would ever get done. And the other side of the coin is true too... action by itself, with no ideas or plans behind it, is completely futile and short lived.
