I'm looking for a word that describes an entity that is the reason for another entity to exist. If that original entity stopped to exist, the other entity wouldn't exist because its right to exist would vanish with it.
for example: citizens to government.
without the citizens, government wouldn't make much sense. Its right to exist comes from the fact there are citizens.
A would be meaningless if B didn't exist. B is the word_i'm_looking_for of A.