The Interface Segregation Principle Is a Corollary of the Dependency Inversion Principle

A first companion paper proves the structural biconditional between ISP and per-client DIP: applying DIP’s ownership clause once per client–provider edge produces exactly the interfaces that ISP prescribes, and conversely. That proof identifies five elements — provider, two clients, two interface slices — but does not determine where they belong in the module structure. A second companion paper shows that no partition of these elements satisfies all of Martin’s packaging principles simultaneously: CCP and REP are jointly unsatisfiable for any multi-client provider configuration, and the tension triangle offers no escape. This paper applies the Independent Variation Principle (IVP) to the same configuration and obtains two results. First, the forward direction of the ISP–DIP biconditional (DIP implies ISP) is formalized in IVP terms. Second, IVP’s modularization methodology — identify elements, identify change drivers, apply the axioms — yields a unique partition: each interface slice resides with its client, because they share change drivers. For a given change-driver assignment, IVP resolves the packaging question that SOLID leaves underdetermined.