What Is Design? A Unified Theory Grounded in Predictive Models and Metacognitive Experience

What is design? Existing theories each address part of this question — Simon defines the scope ofdesign activity, Alexander specifies evaluative criteria, Schön characterises competent method — butnone separates four questions a complete theory must answer: what design activity is, what gooddesign is, how good design is achieved, and how design capability is acquired. This paper proposes aunified framework that addresses all four. Design activity is the cognitive activity of creating formthat realises requirements understood as constraints (Juarrero, 1999, 2023). Design capability isenabled by an internal predictive model, acquired through trial and error. Design evaluation operatesthrough metacognitive experience — the family of felt, nonconceptual, process-based evaluativesignals identified by Efklides (2006) and grounded in Proust's (2013) account of proceduralmetacognition. Design development is the process through which the predictive model is acquiredand refined. Within this framework, Simon's satisficing is search termination at sufficiently lowprediction error, Alexander's misfit detection is the feeling of wrongness, and Schön'sreflection-in-action is predictive model updating triggered by metacognitive experience. Implicationsare derived for design education, human–AI collaboration, and organisational knowledgemanagement.