Ku, the void, emptiness or heaven, is the fifth and highest of the Godai, the five elements of Japanese esoteric Buddhism, and it differs in kind from the four below it: rather than a material principle it is the element of spirit, sky and the formless, the emptiness that is not mere nothing but the pregnant void from which form arises and to which it returns, close to the Buddhist notion of sunyata, emptiness. In the gorinto, the five-ring stone pagoda, the void is the jewel or flame-shaped form at the very top, crowning earth, water, fire and wind.
The five came to Japan through esoteric Buddhism from India by way of China, systematised in the Shingon school. Miyamoto Musashi ended his Book of Five Rings with a short, elusive chapter on the void, the state beyond technique. The Godai are an old and living tradition, and the void as their summit, the formless ground of the elements, descends from the Buddhist teaching rather than from any modern esoteric construction.