Argument by analogy
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"Pork, the other white meat."
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In an argument by analogy it is claimed that if two things have certain characteristics (A) in common, then they are also probably have one or more additional traits (B) in common. When done well the argument can lead to an increase in knowledge. For example, Charles Darwin noted the similarities between animal and plant breeding (artificial selection) and natural selection in developing his theory of evolution. But when done poorly or deceptively it can mislead, and is a leading propaganda tool. Here is the basic form:
Sam likes Bach, Beethoven, Brahms and Bruckner.
I now Sally likes Bach, Beethoven and Brahms. She will probably like Bruckner also, so I will buy this CD for her.
In many cases the additional trait(s) are simply implied. Calling pork "the other white meat" implies that like chicken and fish it is better for you than red meat. The advertisement can convince at least same of the viewers, without any data to support it.
Evaluating analogies:
There are a number of things to look for in trying to decide if an analogy is strong or weak:
Number and quality of examples.
Suppose Jones is trying to discredit Smith, her opponent in the mayoral race, by showing him to be an evil person. She compares him with Hitler, who was certainly a bad person. But any characteristics might be unique to Hitler, and not to evil people in general. It would be a stronger argument if she also compared him with Stalin and several other evil people. If, on the other hand, Smith was compared with someone no one had ever heard of it would be a weaker argument, because people would assume that the example had been carefully chosen.
Counter-examples left out
If there are examples that have A in common but not B this weakens the argument. For example, if Jones notes that Hitler, Stalin and Smith all have moustaches, her argument could be destroyed by pointing out the large number of nice people, like kindly Dr. Brown, who also have mustaches. For more see The Galileo gambit.
Number of similarities:
Consider how many traits there are in common. If there are only one or a few, as in the pork example, the argument is much weaker than if there are more of them.
Relevance:
More important than just the number of similarities is how relevant they are. A comparison between the nutritional values of pork and chicken would be much more valuable in determining the healthiness of pork than simply noting that they are both white. Noting that Sam and Sally both drove red cars, have dogs named Rex, like to take long walks on the beach, etc. would tell us nothing about their possible shared musical tastes.
Dissimilarities
Are there relevant traits that the example(s) and the subject don't have in common? For example, in a widely quoted speech science fiction author Michael Crichton claims that the environmental movement is a religion, based on a few claimed similarities with Christianity. But Chrichton leaves out important differences. There is no supernatural being in environmentalism, no belief that the Son of God came to earth and was crucified, no belief in an afterlife, all major tenants of Christianity.
Questionable similarities
In many cases the claims of similarities themselves are weak. For example, Crichton claims that eating organic food is like taking communion. But they aren't. Taking communion is taken as a religious activity, it represents the blood and flesh of Christ. There is no religious significance to eating organic food. It is claimed to be healthier, and also better for the environment because no pesticides were used in its production.
Questionable other traits
That the example(s) have the other trait(s) is often simply taken as a given. Some people, for example, would argue that eating chicken or fish is not all that helthy. In his speech Crichton simply assumes that members of a religion believe blindly, with a beliefs that cannot be changed by facts. But most believers would claim that their beliefs are shaped by facts, and are subject to change.
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Written by Jim Norton
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