The five are the 五大 (godai), the five great elements of Indian and Buddhist physics, of which everything is made; the first four are the old four, and the fifth, 空, was added to them as the place in which matter exists. It is space, not the sky the character means in ordinary Japanese, and not the Buddhist emptiness written with the same character.
The readings given here are the Sino-Japanese ones; the everyday readings mizu, hi, kaze and sora are the characters' and not the doctrine's. The five-ring stupa in every Japanese graveyard is the five stacked from the earth up.
Void is Musashi's word for it; space is the doctrine's, and the five here carry both.
The Five Elements are most commonly known thanks to Miyamoto Musashi's The Book of Five Rings.
The five are a Buddhist doctrine from India; the earliest Japanese example of their name, 五大, that the 日本国語大辞典 gives is Kūkai's dedication of 821, and in 1645 Miyamoto Musashi named the five scrolls of his treatise on strategy for them.
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