Recurrence has been shown to arise as a structural default in systems that exhibit partial conservation, feedback coupling, and boundedness. Under nonlinear conditions, repeated recurrence produces structural drift, motivating the relation Ψ=∂S/∂RΨ = ∂S/∂RΨ=∂S/∂R as a measure of structural sensitivity to recurrence. If recurrence is structurally expected across domains, a question emerges for biological cognition: why do organisms primarily perceive sequential trajectories rather than the generative rules that produce them? This paper introduces the concept of structural access, defined as the capacity of a system to detect or modify its own rule space under recurrence. The analysis shows that most biological systems operate in the null regime (Ψ ≈ 0) because state-level optimization within fixed rule classes is energetically efficient, evolutionarily stable, and cognitively tractable. Structural access therefore appears primarily when persistent constraints force a transition from trajectory optimization to rule inspection. The paper links recurrence theory, structural floors, and rule-space transition frameworks into a unified explanation for why biological cognition typically operates within fixed rule classes while only rarely transitioning to structural rule awareness. Part of the KOGNETIK Research Series, a structural operator framework for analyzing recurrence, rule mutation, and structural drift across physical, biological, and cognitive systems. Intellectual Property & Licensing The KOGNETIK Research Series is released under the Creative Commons Attribution–NonCommercial 4.0 International License (CC BY-NC 4.0). All scientific works within the series may be cited, shared, and adapted for non-commercial research purposes with proper attribution. Commercial use—including consulting, advisory services, integration into commercial platforms, monetized training, certification, or system-level deployment—is not permitted under this license and requires a separate written agreement. Full license text:https://creativecommons.org/licenses/by-nc/4.0/ For licensing, partnerships, translations, or applied development inquiries:research@kognetik.dehttps://www.kognetik.de ORCID: https://orcid.org/0009-0000-8544-4847 Kognetik Series Information KOGNETIK — Minimal Operator Definition of Reflexivity (Ψ = ∂S/∂R) Reflexivity as structural rate-of-change:Ψ = ∂S/∂R measures structural drift under recurrence. Process, not state:Reflexivity specifies a transformation rule rather than a content or level. Domain-independent operator:Applicable across biological, cognitive, artificial, social, industrial, and geophysical systems. Non-ascriptive and empirically testable:Ψ enables comparative analysis of systems via observable structure and recurrence. Higher-order phenomena as specifications:Learning, adaptation, consciousness, governance, and identity are structured regimes of Ψ.