📓 The practice of medicine basically depends on three general concepts: an individual patient image, the individual disease course of this patient, and an expectation of the future development of the disease. Physicians use heuristics for the cognitive processing of these concepts. Basic clinical heuristics are put into three algorithms: a history-based-learning algorithm, a diagnostic-inferencing algorithm, and a prognostic-planning algorithm. A proto-theory of clinical reasoning for practicing physicians is proposed.