Is a Zen koan really an unanswerable riddle?

Not in Zen's own account of its practice. The popular Western idea is that a kōan is an unanswerable question, or a meaningless or absurd statement. But in Zen practice a kōan is not meaningless, and not a riddle or a puzzle, and teachers do expect students to present an appropriate response. The main goal of kōan practice is to achieve kenshō, to see one's buddha-nature.

The word helps. The Chinese gōng'àn originally referred to a "public record" or the case records of a public law court in Tang dynasty China. A kōan is, in this sense, a public case: a recorded saying or encounter of a Zen master, which later masters comment on. The same word appears in the Chinese title of Celebrated Cases of Judge Dee, a famous detective novel.

One famous case: a monk asked the master Zhaozhou whether a dog has Buddha-nature. Zhaozhou said "Wu", which means "no", or "without". Another case gives a longer version in which Zhaozhou says "yes" to the same question asked by a different monk. Wumen, who published the Gateless Gate, told students to concentrate themselves into this "Wu", making their whole body one great inquiry.

Historians add a warning: the stories are not what they seem. Mario Poceski says the master-student dialogues are not historically reliable and have little or nothing to do with the real Tang-era masters in them. Albert Welter calls the idea that they record real back-room encounters an artifice. Even inside Zen, the master Bankei called kōans "old wastepaper".