Deductive and Inductive Reasoning

Deductive reasoning begins with a general assumption taken to be true and draws a specific conclusion from it — reasoning from the general to the particular. In the platform example, the general assumption is that 'people run on the platform only when they are getting late for the train'; combined with the observation that this man is running, deductive reasoning concludes that he is getting late for the train. The chapter is careful to flag the main pitfall: people frequently assume their starting premise is true without actually verifying it, and if the underlying assumption does not hold (people can run on a platform for several other reasons too), the conclusion, however logically derived, ends up invalid or simply wrong. A cartoon mouse who reasons, 'All cats have four legs, I have four legs, therefore I am a cat,' illustrates exactly this trap — the reasoning follows a valid-looking form while resting on a false premise, so the conclusion is false.

Inductive reasoning works the other way: it is reasoning based on specific facts and observations, moving from the particular to the general. In the same railway example, watching what the man actually goes on to do — entering the train compartment and returning with a bag — and concluding from that specific observation that he had left his bag in the train is inductive reasoning; the main risk here is jumping to a conclusion before all the relevant facts are known.

Reasoning, seen this way, is the process of gathering and analysing information to arrive at a conclusion, which makes it a form of problem solving in its own right — the goal being to determine what conclusion the given information supports. Most scientific reasoning is inductive: scientists (and lay persons too) consider a number of instances and try to work out the general rule that covers them all, in essentially the same way a person applies inductive reasoning while planning a play or a project by drawing on how similar tasks were handled before. …