A Candidate Structural Law of Systems: Coupling, Demand, Capability, and the Generative Interlock

This preprint proposes a candidate structural law of systems based on three distinct, interdependent generating variables: coupling (Γ), demand (D), and capability (C). Coupling determines how processes act within a system and across its boundary, and where their effects land. Demand specifies what must be met for the system to persist or perform. Capability specifies what is available and in which forms. Their joint relation determines the possible process rates through one compatible allocation of capability that meets all applicable demands together. The proposed generative interlock describes how present possibilities give rise to later possibilities. Interaction selects realized processes without adding to or removing from the possible-rate set. These processes act through coupling, and their consequences reshape installed coupling, demand, or capability. The next possible rates arise from the reshaped three-variable structure. Every installed reshaping changes the possible-rate profile under at least one admissible surrounding condition or a declared later response. The structural form remains fixed; evolution is the resulting trajectory, not an additional generating variable. The paper specifies destructive tests of the proposed structure, including role-merging, frozen-role, interlocked-response, and completeness tests. A controlled reaction-network realization demonstrates that equal total fuel does not determine equal finite-horizon continuation when distinct fuel forms are coupled to different routes. It also exhibits feedback through coupling and capability and maintenance through installed reorganization. Demand remains fixed in this realization. The candidate is compared with viability theory, Dynamic Energy Budget theory, flux balance analysis, Chemical Organization Theory, Petri nets, Kappa models, active inference, adaptive networks, and assembly theory. The paper identifies existing correspondences, states explicit failure conditions, and distinguishes demonstrated mathematical results from unresolved claims of generality. The proposed three-variable law is not presented as a proven universal principle. Its validity and irreducibility remain open to the specified theoretical and empirical tests.