The Sandbox Paradox

This conceptual paper presents a methodological model for examining how the choice of system boundaries affects causal interpretation. It distinguishes the physical or functional interface from the interpretive domain selected by the analyst and from the observation rule applied to the system. Temporal change is represented through the evolution of domain membership and boundary behaviour. The paper defines Boundary-Induced Indistinguishability (BII) as the specific case in which two different mechanisms appear identical because the selected domain excludes the information needed to distinguish them. A justified alternative domain can resolve this indistinguishability while leaving the physical system and the observation rule unchanged. The proposed procedure supports the assessment and reporting of boundary stability, boundary sensitivity, observation sensitivity, and indeterminacy. Its application is illustrated through a formal operational example involving apparent generation, delayed transfer, and accounting across nested domains.