Deductive logic produces true conclusions from true premises. It deals with the necessity of certain conlusions based on certain premises. Inductive logic, which is fuzzier, deals with how certain premises support certain conclusions.

Included in deductive logic are syllogisms and conditional reasoning.

Whether humans are built to use deductive logic is an interesting question. We certainly use it implictly when parsing sentences, but it is difficult for us to apply it explicity and abstractly without training. We tend instead to use inductive logic in real worldsituations. Which actually makes more sense, from an evolutionary perspective -- inductive logic, while fallible, is more powerful.

Humans built with a bias for deductive logic would probably have died out.

"Hmmm. Now that's a tiger. If all tigers have teeth, and all teeth are dangerous, then this tiger is probably dangerous. But how do we know that ALL tigers have teeth? How can I know that this tiger will -- AAAAHHHHHH!!!!!...."

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