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Willamette Law Review


This paper uses the work of C.S. Peirce to explore legal reasoning by analogy. Peirce divided reasoning into three basic forms: deduction, induction, and abduction. Deductive reasoning discloses conclusions that necessarily follow from the premises. Inductive reasoning gives support to statements by generalizing from the characteristics found in samples. Abductive reasoning - a concept that Peirce originated - produces explanatory hypotheses. The three types of inference vary as to security (how certain we are that a conclusion follows from the premises) and uberty (how fruitful the reasoning is in producing new knowledge). Deductive reasoning has high security, because the conclusion necessarily follows from the premises, but low uberty, for the same reason. Abductive reasoning has high uberty, because it creatively produces explanatory hypotheses, but low security, because such hypotheses may be falsified when tested. Induction falls between the other two forms; compared to abduction, its reliance on regularity increases its security, but restricts its uberty. The classic formulation views reasoning by analogy as induction; because one or more base entities with certain characteristics have an additional characteristic, we conclude that another entity with the initial set of characteristics also has the additional characteristic. Peirce, however, saw analogy as a combination of induction and abduction, which would both make the process of reasoning by analogy more complex and add the properties of abductive reasoning (for example, its lower security and higher uberty). Applying Peirce's framework to legal reasoning provides a powerful analytical device for assessing the strengths and weaknesses of analogical arguments. The paper also analyzes various writings on legal reasoning by analogy, using the framework sketched above.

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