Physical-Source Regress and Boundary Enlargement: A Structural No-Go Result for Local Physical Initiators

A physical source can be external to a local physical subsystem: a finger can push the first domino because the finger is outside the domino row. But a physical outside source cannot terminate the regress of initiation. The physical-source regress law is simple: an added physical source is an expansion of a closed physical system. If the proposed outside source is itself physical, admitting it as a cause adds it to the relevant physical description and enlarges the system under explanation. The onset question then reapplies to the enlarged system. This paper proves the resulting no-go: local physical initiators do not terminate the regress of initiation. The paper introduces the distinction between subsystem-externality and description-externality, illustrates the result with the domino case and the case of animal action, and applies it across neuroscience of volition, cognitive control research, and cosmology. It depends on and extends the cited initiation scaffold; it does not re-derive it.