Augmentation or Substitution? External Systems and the Boundaries of Agency

External systems are commonly understood as tools that extend human agency. Calculators, navigation systems, and algorithmic assistants are used to enhance our capacities by offloading cognitive labour. This paper argues that this view is incomplete. External systems do not merely assist agency; they either participate in it or displace it. The distinction turns on whether such systems integrate with the agent’s own processes of coordination or substitute for them. Augmentation occurs where external systems support and remain available to the agent’s integrative activity. Substitution occurs where they perform integrative work in ways that are not available for coordination, revision, or mutual constraint within the agent. On this account, the central question is not whether external systems improve outcomes, but whether they preserve the agent as the locus of integration.