What is design? Existing definitions describe what design achieves (Simon's transformation toward preferredstates) or how it evaluates (Alexander's detection of misfit), but none explains how design capability emergesin the first place. More fundamentally, existing theories may share an unexamined assumption: that designoperates at the level of linguistic cognition. This paper questions that assumption and proposes a unifiedfoundation.We argue that design is the acquisition of an internal predictive model that achieves fit with constraintsthrough trial and error—a process that is pre-linguistic in origin. When a child discovers that pressing a pencilharder produces darker lines, they are acquiring a predictive model before they have words for "pressure" or"darkness." This primacy of experience over language, supported by developmental psychology,neuroscience, and evolutionary biology, reveals that design capability is more fundamental than existingtheories recognise.This definition unifies previously disparate frameworks. Simon's satisficing becomes search for what fits.Alexander's misfit detection becomes prediction error detection. Schön's reflection-in-action becomespredictive model updating. Juarrero's enabling constraints become the requirements that define the space ofpossibilities. What appeared to be competing theories are revealed as descriptions of different aspects of thesame underlying process.The framework also reconceptualises the relationship between requirements and constraints. Requirements arenot separate from constraints—they are constraints that define the space of possibilities. Thisreconceptualisation, drawing on Juarrero's work on constraints, resolves longstanding ambiguities in designtheory.Finally, we argue that the concept of "design documents" is historically contingent—an adaptation to thebandwidth limitations of print-based knowledge transmission. In the era of large language models, theexternalisation of How becomes less necessary; what remains essential is the designer's predictive model forevaluating What and Why. This has profound implications for design education, which has often confused themanufacturing of design documents with design itself.
Ishibashi Ryuhei (Tue,) studied this question.