There are two principle forms of arguments and reasoning: deductive and inductive. Deductive reasoning starts with general principles to a conclusion about a particular case. With deductive reasoning if the premises are true then the conclusion must be true as well. Deduction is ideal for scientific reasoning. But in medicine we often start with symptoms and signs in an individual patient and work from there – i.e. we start with a particular case and then apply general principles. This is inductive reasoning. Unlike deductive reasoning, inductive reasoning involves probability. The truth of the premises does not guarantee the truth of the conclusion. When clinicians confuse deductive and inductive reasoning, what results is called a ‘logical fallacy’.
In situations that involve uncertainty, i.e. medicine, deductive reasoning does not work. Philosopher Charles Pierce described another type of reasoning which is a blend of induction and deduction and probably intuition – he called it abductive reasoning. Abductive reasoning is used frequently in medicine. It is a process of choosing the hypothesis that would best explain the available evidence.
Kathryn Montgomery, in her book, ‘How Doctors Think’ (OUP, 2006) writes, ‘Physicians do not reason as they imagine scientists do. Whether making a diagnosis or deciding on treatment, physicians when face to face with a patient do not proceed as they and their textbooks often describe it: deductively, ‘scientifically’. They use case-based reasoning instead, in a similar way to detectives … Physicians rely on the narrative organisation of details in a reasoning process that starts ‘bottom up’, or inductively, from the particulars, and then circles between particular observations and general rules, fitting the details to the patterned whole and testing them in the light of known generalities until a workable conclusion is reached. This is a practical, interpretive way of reasoning in an uncertain world - neither deduction nor induction but abduction’.