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Deductive reasoning, also deductive logic, logical deduction or, informally, "top-down" logic,〔(Deduction & Induction, Research Methods Knowledge Base )〕 is the process of reasoning from one or more statements (premises) to reach a logically certain conclusion. It differs from inductive reasoning or abductive reasoning. Deductive reasoning links premises with conclusions. If all premises are true, the terms are clear, and the rules of deductive logic are followed, then the conclusion reached is necessarily true. Deductive reasoning (top-down logic) contrasts with inductive reasoning (bottom-up logic) in the following way: In deductive reasoning, a conclusion is reached reductively by applying general rules that hold over the entirety of a closed domain of discourse, narrowing the range under consideration until only the conclusion(s) is left. In inductive reasoning, the conclusion is reached by generalizing or extrapolating from, i.e., there is epistemic uncertainty. Note, however, that the inductive reasoning mentioned here is not the same as induction used in mathematical proofs – mathematical induction is actually a form of deductive reasoning. ==Simple example== An example of a deductive 'syllogism': # if 'nature' is good... # and animals 'exist by nature'... # then 'animals' are good. 'All' premises must begin with a 'subject', in this case 'nature', and then always must be 'followed' by a certain predicate. here, the predicate of the first premise is: 'is good'. The second premise states that "animals", again, a 'subject', is followed by the predicate: 'exist by nature'. the third 'subject', again, 'animals', is followed by the predicate: 'are good'. 'all' premises and 'all' conclusions must 'begin' with a 'subject', whereas 'all' premises and 'all' conclusions must 'end' with a predicate. a valid deductive-syllogism must 'always' have a total of 6-terms. no more ; no less. in this particular argument, the 6-terms are: 'nature', 'animals', 'animals', 'is good', 'exist by nature' and 'are good'. a correct syllogism must always have a combined-total of 3 'subjects', as well as a combined-total of 3 'predicates'. no more ; no less. otherwise, the entire syllogism, including the conclusion, will 'always' be incorrect. in this particular-case, the 3 subjects are: 'nature', 'animals' and 'animals', whereas the 3 predicates are: 'is good', 'exist by nature' and 'are good'. in addition to having 6-terms only, as well as having no more and/or no less than 3 subjects and 3 predicates, a correct-syllogism must also have 2 'differing' subjects as well as 2 'differing' predicates. again: no more ; no less. otherwise, the syllogism will ultimately be false. the 2 differing-subjects in this particular syllogism are: 'nature'/'animals', whereas the 2 differing-predicates are: 'are good'/'exist by nature'. this particular-syllogism is correct, in that it has a sum-total of 6-terms, 3 total-subjects, 3 total-predicates, 2 differing-subjects and 2 differing-predicates. this means that a conclusion exists, although it's not always the 'correct' conclusion. Conclusion: 'both nature and animals exist and are good'. this particular-conclusion is 'correct' in that it simply makes sense based on the information residing within this particular syllogism. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Deductive reasoning」の詳細全文を読む スポンサード リンク
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