This chapter examines the classical paradigms of thinking about medical problems in terms of classical logic and probability, as well as some modern competing theories in two areas. Medicine is needed to be logical in order to better understand it and to use it effectively to the benefit of the patient. If asked about philosophy in medicine, many medical students will mention ‘medical ethics’. The logic of medicine is a system of thought and reasoning that governs understanding and decisions in clinical and community care. The chapter deals with the application of informal logic to medicine. Logic and medicine, in general, have assigned slightly different meanings to deduction and induction. Essentially, there are three possible ‘operating systems’ of reasoning in medicine: Probability theory and its applications; chaos theory and its uses in clinical research; and fuzzy logic and fuzzy sets theory in handling imperfect or hard to interpret data.