<p>Diagnosis is a foundational task in clinical medicine, yet its reasoning structure remains under-explained in medical education. This paper presents a five-step model of diagnostic reasoning that analyses the epistemic foundation of diagnosis: namely, the structured process by which clinicians construct coherence under constraint. The model describes diagnosis as a disciplined convergence from presenting data toward an explanatory frame that is internally consistent, pathophysiologically plausible, and resistant to contradiction. Grounded in coherence-based epistemology and abductive inference, the framework clarifies how clinicians move from presentation to working diagnosis through five recursive phases: problem framing, perspective construction, constraint application, stabilisation, and clarification. It integrates, rather than replaces, existing approaches—such as dual-process theory, illness script development, and Bayesian updating—by making their underlying reasoning structure explicit and teachable. The model serves as a scaffold for reasoning transparency, structured feedback, and reflective practice. It provides coherence criteria for assessing diagnostic thought and supports formative instruction across simulation, objective structured clinical examination (OSCE), and bedside contexts. While awaiting empirical validation, the present framework offers a theoretical foundation for studying how diagnostic coherence develops and can be taught. In this account, diagnosis is not defined by classification or probability alone, but by the clinician’s capacity to construct and sustain an explanatory model that coheres with evidence and contextual constraints.</p>