Usually, people employ analogies to help establish a relatively controversial claim. They do so by trying to establish a similar claim in a less controversial case, which they claim is analogous.
So, the set-up for any argument by analogy is always the same. First, an analogy is drawn between the "controversial" case and the "uncontroversial" case. Second, a claim is made about the "uncontroversial" case. Using the picture I drew in class, the set up is as follows:
Based on this set-up, the conclusion is that, by analogy, the same thing is likely to be true in the controversial case.
1. The "controversial case" is analogous to the "uncontroversial case".Again, using our picture, the argument is as follows (with the conclusion in blue):
2. X is true of the "uncontroversial case"
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C. So, by analogy, X is true of the "controversial case" as well
Now, it should be noted that analogical arguments are not deductively valid. The premises can be true without the conclusion being true. However, we ordinarily think that a good argument by analogy gives us strong reason to believe the conclusion.
In case the preceding discussion was a bit abstract, consider the
following
example:
One of the ways that scientists test whether various drugs will be
safe and effective on humans is to observe the results of the drug on
animals.
Of course, the success or failure of the drug on some animal doesn't
guarantee
that it will have the same results on humans. (Again, arguments by
analogy
are not deductively valid.) But a
comprehensive
test on animals gives us good reason to think the effects will be
similar
on humans.
So, let's say that the latest drug by Drug Co. has terrible
side-effects
on a group of lab rats. This causes the FDA to not approve the drug. I
take the FDA to be reasoning roughly as follows:
1. Humans are analogous to lab rats. (We are similar in a number of respects.)
2. Drug Co's latest drug caused terrible side-effects in the test group of lab rats.
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C. So, by analogy, the drug could have terrible side-effects on humans as well.
Remember that all arguments by analogy involve two claims:
1. The "controversial case" is analogous to the "uncontroversial case".So, to object to such arguments we can always question either of these claims.
2. X is true of the "uncontroversial case"
First, we can question the analogy. As an intentionally bad example of analogical reasoning, consider the following:
1. Iced tea resembles whiskey.This is, of course, a bad argument. It is bad because the analogy between iced tea and whiskey (premise 1) is very weak. While iced tea does resemble whiskey in visual appearance, there are a number of relevant dissimilarities. They smell different, taste different, and, most crucial in the context of this argument, one contains alcohol while the other doesn't.
2. Iced tea doesn't impair my ability to operate a motorized vehicle.
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C. So, by analogy, I can knock back a fifth of Jack Daniel's and get behind the wheel.
The general lesson here is this:
One way to object to an argument by analogy is to question the analogy. This is done by pointing out relevant disanalogies between the two cases.Alternatively, we can always question the claim being made about the "uncontroversial case". If we have reason for doubting this claim, then regardless of how strong the analogy is, we have no reason for accepting the concluion. So:
Another way to object to an argument by analogy is to question the claim being made about the "uncontroversial case".
