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deduction, in logic, form of inference such that the conclusion must be true if the premises are true. For example, if we know that all men have two legs and that John is a man, it is then logical to deduce that John has two legs. Logicians contrast deduction with induction, in which the conclusion might be false even when the premises are true. Deduction has to do with necessity; induction has to do with probability. The famous Aristotelian syllogism is one species of deductive reasoning, which was greatly extended by the development of symbolic logic.
See R. J. Ackermann, Modern Deductive Logic (1971); P. J. Hurley A Concise Introduction to Logic (1985).
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