Zen kōans are stories, statements, or questions for which a solution is sought, but no such solution or answer exists. For example:

Such propositions defy our usual way of thinking, and any attempt to arrive at a definitive answer can be daunting or impossible. Nonetheless, there is value in asking them repeatedly and observing what the mind conjures in response. Sometimes the best approach to take with these propositions is simply to experience them, and give up on attempting to reach a rational reply. This attitude can be encapsulated in the concept of Mu.

Mu’s meaning, at least as popularized by Robert M. Pirsig in Zen and the Art of Motorcycle Maintenance, is “No class; not one, not zero, not yes, not no," or in short “unask the question”. In other words, the inquiry does not fall into the category of questions with a concrete answer. Rather, it is intended as a tool to exercise the mind, with the ultimate goal of completely bypassing thought and discovering one’s true nature.