The Necessity of Structural PSR

This paper establishes that structural PSR — the requirement that the universe’s existence as a complete structured whole have an explanation — is the necessary and unique formulation of the Principle of Sufficient Reason adequate for cosmic explanation. The argument proceeds via two functional constraints derived from what cosmic explanation must achieve. An adequate formulation must individuate this universe from alternatives: an explanation compatible with every possible universe explains none in particular. And it must preserve internal contingency: an explanation that necessitates every fact within the universe generates modal collapse, exceeding what the explanatory problem requires. Weak PSR fails the first constraint: its explanation is satisfied by any universe whatsoever and therefore fails to explain this universe in particular — it explains existence in general, not this existence. Full PSR fails the second by generating modal collapse within the cosmic domain, imposing absolute necessitation where the explanatory problem warrants only closure-level necessitation. Structural PSR uniquely satisfies both constraints. The boundary it draws between the universe as a complete structured whole and the contingent facts within it tracks the distinction between boundary explanation and nomological explanation already operative in scientific practice — a principled joint, not a gerrymandered line. The result is not a position in a debate about PSR formulations. It is a consequence of what cosmic explanation requires.