An argument by analogy takes place when the arguer:
- Has a point to prove about something
- Gives you an analogy for the thing
- Points out that the analog contains the feature they are trying to prove exists in the original
- Concludes the feature in the analog must also exist in the original.
The flaw in this method of argument
The flaw is simple: the analogy always breaks down somewhere. If the analogy breaks down, the conclusion the arguer is trying to draw from the analogy just doesn’t follow. That’s all there is to it. Argument by analogy is rarely as good as an argument by logic, evidence or facts. Clearly if the arguer had any logic, evidence or facts to support his case he would present them. That he resorts to argument by analogy shows his argument is probably devoid of logic, evidence or facts.
The most persistent users of argument by analogy are ID creationists. For example, Michael Behe is especially fond of his Mount Rushmore analogy:
For example, unintelligent physical forces like plate tectonics and erosion seem quite sufficient to account for the origin of the Rocky Mountains. Yet they are not enough to explain Mount Rushmore.
Of course, we know who is responsible for Mount Rushmore, but even someone who had never heard of the monument could recognize it as designed.
So by analogy, life on Earth must also have been designed. He goes on to invoke William Paley and his watch analogy – essentially the same argument only with a watch instead of Mount Rushmore. The obvious place the analogy breaks down is that neither Mount Rushmore nor a watch can have offspring, while living things regularly reproduce themselves. Since this reproduction (with minor changes) is fundamental to the theory of evolution, and since it obviously doesn’t work with Mount Rushmore, Behe’s analogy is bogus. The only startling thing in my view, is how ID proponents like Behe can still use this lame analogy when it is so obviously bogus and has been debunked so many times.
Michael Egnor (another creationist) uses argument from analogy in his plea for dualism - ‘Verizon Deniers’ Find a Cellphone. He compares the human mind to a cell phone, which (as we all know) picks up signals from elsewhere. That is, we know the people we hear on the phone don’t actually live inside the phone – the phone is just a receiver of the voices. Egnor concludes that the human brain is the same – the mind exists elsewhere (in some unspecified non-material realm), and the brain is just a receiver for the mind’s thoughts. Of course, we know that if we placed the cell phone in a faraday cage that is impervious to radio waves, the signal would be lost. That is one place the analogy breaks down, since there is no equivalent way to block the “signal” (analog or digital – ha ha), from the “mind” to the brain. We also know that damage to specific areas of the brain can produce specific changes in personality or other brain functions, while damage to specific areas of the phone produce no analogous changes in the message received. (Usually damage to the phone produces either no change or the phone stops working completely.) So right there we have two places the analogy breaks down, and we can dismiss the argument. It is notable that Egnor produces no actual evidence for this rather extraordinary claim of his. (Orac did a fine and more detailed take-down of Egnor’s argument, as did Steven Novella and others.)
Correct uses of analogies
I believe there are two correct uses of an analogy in a discussion. The first is to explain something that is complex and perhaps technical, in language that is simpler or within the recipient’s experience. As an example, I modestly (well no, not really) present the Beatles analogy I used in the Lost Tomb of Jesus post. James Cameron used the analogy of some future archaeologist finding a tomb in Liverpool, marked with the names John, Paul, George and Ringo, and concluding he had found the lost tomb of The Beatles. I wrote that, by analogy, Cameron had really found a tomb with the names John, Paul, George and Britney, and concluded Britney Spears was the drummer in The Beatles. (Bear with me – you have to read the whole post.) But my analogy wasn’t used to make the case – I had done that by examining the statistics and showing where they were wrong. The analogy was to help explain it.
The other correct use of an analogy is to get around a mental block – where someone simply can’t conceive of the point you are making so you make an analogy they might be able to think about. An example would be to explain to a theist, how an atheist simply has no belief in God. A religious person possibly couldn’t even conceive of there being no God, so instead you present Bertrand Russell’s teapot analogy. Here, he believer is asked to consider if he believes there is a teapot in orbit between the Earth and Mars, and when he realizes he can’t prove there isn’t one he might understand why the atheist has no belief in God and why the burden of proof is upon the person saying God (or a teapot in orbit), exists, rather than upon those saying they don’t. Note that the teapot analogy doesn’t prove the point, but it might help the theist understand the idea.