Rereading Popper’s “The Logic of scientific discovery”, at his 120th anniversary, brings some thoughts regarding the diagnostic process and decision making in medicine from the viewpoint of the classical scientific method. In recent years physicians are increasingly becoming technical experts who base their decision-making on uniform criteria, guidelines and classifications but unfortunately have moved away from understanding the basic concepts in the philosophy of science. This raises an ethically and philosophically important issue; what does a medical diagnosis mean? Is this an absolute or a relative truth? The implications of this question are enormous in terms of prognosis and treatment. Both patients and physicians should be educated about the nature of the diagnostic process.