The understanding of the scientific basis and the theory of knowledge are surprisingly heterogeneous in practical and clinical medicine. It is frequently influenced or based on the philosophical theory of critical rationalism founded by Sir Karl Popper. Because the theory of knowledge and the understanding of scientific truth is the central basis for cautious and good clinical practise it is necessary to discuss both points to avoid unscientific auto-immunisation against critique in a type of medicine that regards herself as science-based. Evidence-based medicine would not be possible without interpretation and explanation of existing data into the individual treatment context. Besides an inductive or deductive logic the historical and situative side-conditions of the gathering of knowledge and of experiments are of central importance for their interpretation and their relevance in clinical practice. This historical and situative context warrants reflection but must also be paid attention to in the reflections on medical ethics.