Foundation's Edge

He was the analog—the mirror image—of the Mayor of Terminus, who ruled over the First Foundation. But they were different in every respect. The Mayor of Terminus was known to all the Galaxy and the First Foundation was therefore simply «the Foundation» to all the worlds. The First Speaker of the Second Foundation was known only to his associates.

And yet it was the Second Foundation which held the real power. The First Foundation was supreme in the realm of physical power, of technology, of weapons. The Second Foundation was supreme in the realm of mental power, of the mind, of the ability to control. In any conflict between the two, what would it matter how many ships and weapons the First Foundation controlled, if the Second Foundation could control the minds of those who controlled the ships and weapons?

But how long could this remain a secret?

From Foundation’s Edge