One Structure, Inside and Out: Experience as the Content Anchor of Structural Realism

Newman's objection holds that a theory attributing only structure to the world says almost nothing, since any domain of sufficient cardinality can be arranged to instantiate any abstract structure. Structural realism has answered with Lewisian naturalness, causal reference-fixing, Ramseyan humility, and reformulations that mix non-structural vocabulary into the theory; each candidate posits its anchor, supplies no content, renders the anchor unknowable, or concedes that something other than structure carries the content. The Russellian-monist answer — experience supplies what pure structure lacks — has a virtue no rival shares and a defect that has kept it schematic: its anchor is given rather than posited, and it names no structure. This paper makes the anchor specific. The space of orientations an embodied perspective acts within is SO(3) ≅ RP³, and its fundamental group Z₂ is the topology that spinorial matter registers, exhibited mechanically by the belt trick and as a phase reversal in neutron interferometry. One structure appears on the inside, as the orientation manifold experience navigates, and on the outside, as the double cover governing fermions. A neutral-monist identification stronger than Russell's takes experiential structure and physical structure to be one structure. The identification turns the exhibit into a content anchor: it supplies domain content beyond cardinality and fixes reference ostensively, meeting both faces of the anchoring demand. It is advanced as an inference to the best explanation, argued by comparison with the standard dischargers, and the argument degrades gracefully: a reader who withholds the identification keeps a structural parity that stands on its own. Taken realistically, and with its extension from orientation space to place space stated as a posit, the identification offers RP³ as the natural geometry of actuality. The minimal complex completion comes fixed rather than chosen: CP³, carrying the one involution whose fixed points are exactly RP³.