The biologic energy of life. At puberty those who Establish, turn Gen, begin producing selyn, and those who changeover, become Sime, do not produce selyn.
Simes Need selyn to survive in the way Gens must have calories. Gens produce selyn but use none or imperceptible amounts to run metabolism. The only way a Sime can acquire selyn is from a Gen.
Ancients both produce and consume selyn, but in exactly equal amounts at exactly the same moment, imperceptible and for all instruments, unmeasurable and non-existent. Ancient metabolism runs on calories obtained from food. Ancients would appear to a Sime who is zlinning as if they were simply animals.