Deduction: apply general principle to infer a fact.[Examples from John Sowa, ``The Challenge of Knowledge Soup.'']
Given: Every bird flies. Tweety is a bird. Infer: Tweety flies.
Induction: assume a general principle that subsumes many facts.
Given: Tweety, Polly, and Hooty are birds.
Tweety, Polly, and Hooty fly.
Fred is a bat. Fred flies.
Assume: Every bird flies.
Abduction: guess a new hypothesis that explains some fact.
Given: Every bird flies. Tweety flies. Guess: Tweety is a bird.
Deduction is sound; induction and abduction are not sound, but essential for intelligence.