External Inscription Enabling Joint Comparison: Why Drawing a Problem Lets Two People See It Together

This conceptual paper introduces External Inscription Enabling Joint Comparison, a causal mechanism through which drawings, charts, or other external representations stabilize relations for shared reference. Participants can point to the same elements and compare their readings while reducing what each person must hold privately in working memory. The account assesses claims of shared understanding through fidelity to the relations relevant to a particular use and evidence of shared interpretation. Shared access does not establish a shared reading or agreement. What an inscription omits, and who can draw, edit, or challenge it, can shape the discussion. The paper connects the mechanism to Larkin and Simon’s account of diagrammatic representation and examines Tupaia’s chart, made with members of James Cook’s crew in 1769–1770. Drawing on Eckstein and Schwarz’s reconstruction, it considers how different systems of wayfinding were brought onto one sheet and how later European cartographic conventions shaped its interpretation. How fully the original collaborators understood Tupaia’s system remains uncertain. Part of the Shared Capacity and Learning series within On Human Modeling, the paper examines when a shared representation supports joint comparison. Its framework can inform human–AI research on shared visual workspaces, representational omissions, and how to distinguish common access from compatible interpretations.