Setsuna (ephemeral) (刹那)
Setsuna is one of the Buddhist concepts of time and is the briefest unit of time in Buddhism. It is also known as nen.
Length of setsuna
There are various theories as to the length of setsuna, however it is said that there are 65 setsuna in the space of one click of the fingers, and also in the "Mahavibhasa-sastra" (a Buddhist document) it states that 24 hours = 30 mukoritta (1 mukoritta is 2880 seconds) = 900 robaku (1 robaku is 96 seconds) = 54,000 tasetsuna (1 tasetsuna is 1.6 seconds) = 6,480,000 setsuna and the length of one setsuna is 1/75th of a second.
However, Nagarjuna (an Indian philosopher), originator of Yuishiki doctrine, rejects the concept of determining a specific length of time for setsuna.
Setsuna and Buddhist thought
According to the Sarvastivadin school which is a sect of Theravada Buddhism, human consciousness is the mind repeatedly emerging and ceasing in the period of one setsuna. In response to this, Dogen, a priest in the Soto sect, preached in "Shobogenzo" and 'Hotsubodaishin' (Awakening the Mind of Enlightenment) works that the meaning of seeking enlightenment practicing meditation puts such impermanence first, and because it is always changing, the bad is extinguished and good is born.
Setsuna as a unit
Setsuna is used as a numerical unit in countries that use the Chinese writing system. The value of Setsuna differs across the ages and regions, however nowadays it is 10-18(0.000000000000000001), or one-tenth of a dantsu or ten times a rittoku.
In Japanese, the word 'setsunai' which means painful or distressing has no relationship to Setsuna, and is derived from the Chinese character for setsu which means to feel strongly, as can be seen in the way it is written with different characters than Setsuna..