A word for an entity that gives rise to something else

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.

The pattern:

A would be meaningless if B didn't exist. B is the word_i'm_looking_for of A.

Any ideas?

It may not be exactly "English", but most people use the term raison d'être. True English alternatives include basis, justification for existing, rationale, reason for existing, reason why, and my preferred one for OP's context (if I couldn't use raison d'être) - precondition.

A related term is sine qua non, Latin for "without which, nothing". This may be used to describe an aspect or condition, not just an entity, but I think it shares the sensibility you seek.

Antecedent perhaps. I think that this is something that must exist in order that something else exist. There is a better word on the tip of my tongue but I can't remember it.