Lethani

"What is the Lethani?" Serious. Tempi looked at me for a long momnent, then suddenly burst out laughing. "I do not know. And I cannot tell you." He laughed again. Understatement. "Still we must speak of it."

The Lethani is a Zen-like concept encapsulating the essence of correct human action. It is best understood by the more intuitive and ego-less aspect of the mind. Kvothe often realizes it's intricacies via spinning leaf.