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Reasoning by analogy

Analogical reasoning proceeds from the observation that things which are similar in some respects are probably similar in other respects too.
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We’ve been looking at some broad areas in which effective logical and critical thinking are important, and which generate distinct approaches to the questions they address. Last week, we looked at science. This week, we’ll be looking at law, focusing on three important aspects of legal reasoning which connect to broader themes in logical and critical thinking– reasoning by analogy, law’s distinctive attitude to arguments from authority, and the burden of proof. We’ll start with reasoning by analogy. Reasoning by analogy is central to common law legal systems– roughly those which descend from English law. But don’t worry if you’re from a civil law jurisdiction, analogical reasoning is important outside the law, too.
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And the legal example is a useful illustration of a model of reasoning important in it’s own right. So what does reasoning by analogy involve? When we reason analogically, we proceed from the observation that two or more things are similar in some respects to the conclusion that they’re probably similar in some other respect as well. Not every instance of reasoning by analogy is an argument, by the way. We often use such comparisons simply to explain or illustrate what we mean. But arguments by analogy are common, too. We can put that a little formally like this. One, P is similar to Q in certain known respects. Two, P has some further feature– R. Three, therefore, probably, Q also has the feature R.
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Here’s an example– Double Falsehood is an early play usually attributed to Lewis Theobald. Recently, though, US researchers announced that it was almost certainly written by the bard himself, William Shakespeare. How did they reach that conclusion? Well, they used computers to spot similarities between plays we know are written by Shakespeare– 33 of them– and the disputed play. That’s reasoning by analogy. One, P– the plays of Shakespeare– Hamlet, Romeo and Juliet, King Lear, and so on– are similar to Q– Double Falsehood– in a number of respects. Two, P has the further feature, R– they were authored by Shakespeare. Therefore, probably, Q also has the further feature R– it was authored by Shakespeare, too. We reason like that in everyday life, too.
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If I’m thinking of buying a secondhand car, I might reason– one, my last two cars were Morris Minors– the finest cars ever built, with such and such mileage. Two, they both had the further feature of being reliable. Therefore, probably, a Morris Minor with similar mileage will be reliable, too. And that little domestic example should make clear that not every similarity will be relevant. It would be odd to think that colour of my last two cars was any reason to think a car of the same colour would be reliable. Notice, too, that because we’re in non-deductive territory, we’re interested in the strength of the argument by analogy.
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That’ll depend upon things like the number of comparison cases– it would’ve been better if I were considering five previous cars– the number of relevant similarities– the more the better– and the absence of relevant dissimilarities. In the next video, we’ll see how this applies in the law.
When we reason analogically, we proceed from the observation that two or more things are similar in some respects to the conclusion that they’re probably similar in some other respect as well. We can put that a little formally like this.
  • P is similar to Q in certain known respects.
  • P has some further feature– R.
Therefore, probably,
  • Q also has the feature R.
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Logical and Critical Thinking

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