The Isolation Requirement: Contradictory Demands, Favoured Partition, and Guarded Finalisation in Finite Systems

The first two papers in this series constrain knowing: three scoped limits and a criterion-conditional operating discipline (Paper One), and a tolerance-bounded instrument by which a shortfall can be pointed to under controlled comparison (Paper Two). This paper turns to construction, and this version scopes its claims with care. We identify two computational demands whose information signatures are opposed: a committing, low-entropy execution demand, which isolates a single hypothesis and drives it to a finalised output; and a non-committing, high-entropy vigilance demand, which monitors a wide field without collapsing it. Within a declared resource model—a shared processing plane with an interference term, a finite budget, and stated success thresholds—we argue that a single undivided plane at material interference and saturating joint load cannot serve both demands concurrently: each corrupts the other, and the plane saturates; what follows is a requirement of declared, audited isolation within a tolerance, not an exactness. What follows is not a unique architecture but a requirement of informational isolation, and we classify the mechanisms that supply it: parallel channels, time-slicing under a priority scheduler, multiplexing, and orthogonal state with a read-only check port. Isolation mechanisms that interleave or segregate by construction do not require a finalisation coupling; the two-channel member does, and we state the regimes—continuous concurrent demands, non-preemptible commitments, hard deadlines—in which that member is the resource-favoured choice. Its cost is a specific failure: an isolated committing channel that is fluent while blind, releasing well-formed output that the vigilance channel holds a defeater against. We derive that failure formally as a false accept of a guarded-commit interface, define two interfaces exactly—an external competence contract inherited from Paper One, extended with an escalate-with-deadline state for systems that cannot abstain, and an internal release/abstain/unresolved interface for the coupling—and state the composition law by which guard error and contract error combine, together with the refinement obligation that must be discharged before the internal guard can be claimed to implement the external contract. The lateralised mammalian brain, and its severed-coupling failure, is offered as a structural analogy tested against predictions stated and scored in place, with its patterning of our vocabulary acknowledged; it is not evidence. The argument is held at the confidence its premises permit, every added premise is ledgered, and the whole is offered to be checked.