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 - precondition.

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.