Second Physics: Notes on Vacuum, State, Structure, and Physical Ontology

This paper provides a foundational ontological and conceptual completion of the Second Physics framework. Building upon the dynamical formulation of a structured vacuum and its extended state space, we make explicit a set of essential but previously implicit assumptions required to render the theory physically interpretable, energetically consistent, andoperationally meaningful. In particular, we introduce: (i) a thermodynamic interpretation of vacuum structure and memory, (ii) an operational definition of the structural variable Ξ, (iii) a clarification of the physical scale underlying volume averaging, (iv) a principled distinction between vacuum structure and spacetime curvature, (v) the coupling mechanism between fermionic matter and vacuum structure, and (vi) boundary and stability conditions for a structured vacuum. These notes complete the ontological layer of Second Physics, preparing the framework for predictive modeling and experimental exploration.