Epistemic Structural Realism as an Intersubjectivity Constraint - Observer--equivariance and the public content of physics

This note argues that epistemic structural realism can be understood not merely as a retrospective lesson from theory change, but as an intersubjectivity constraint on scientific description itself. If distinct observers are to count as describing one and the same world, what they describe must be representable in a form stable under admissible changes of perspective. The central claim is therefore methodological: public scientific content is given by invariant residue under such transformations. The paper formulates this idea in a compact observer-equivariance framework. Observer perspectives and their admissible transports are represented by a connected groupoid O, while structural content is represented by a category S. A projection p: O -> S maps perspectival descriptions to what remains intersubjectively stable. In simplified form, this yields a quotient picture O/G ≃ S: not as an ontological reduction of reality to structure, but as a representation of publicly shareable content. On this basis, the paper proposes a precise reading of ESR. Structure is not “all that exists,” but what survives admissible re-description and can therefore function as objective content in science. This also provides a response to Newman-style triviality worries: the relevant structure is not arbitrary abstract relational form, but invariance under physically and epistemically motivated transformations. Brief examples from relativity, quantum theory, and gauge theory illustrate the point. The note concludes by suggesting that this intersubjective constraint helps explain, in part, why mathematics is so effective in physics: mathematical structures are especially suited to expressing exactly what can remain stable across perspective change. This version is a revised and tightened presentation intended as a concise philosophical statement of the claim.