People employ analogies in arguments all the time--sometimes well and sometimes badly. Two of the worst analogies I've seen recently involved comparing 'getting pregnant from rape' to 'getting a flat tire' and 'being subject to higher income taxes' to 'getting raped'. Yikes.
On the other hand, two of the very best arguments by analogy that I've ever seen involved comparing a human fetus to a concert violinist and Irish infants to suckling pigs.
This diary is about argument by analogy--the form of the argument, the ways it can be employed, and the process of evaluating it. This time, in honor (?) of the atrocious right-wing thinking (?) on rape, we'll visit a beautiful and powerful argument by analogy that everyone should know and take time to ridicule one of the worst.
Analogy, in and of itself, is just a device of course, and as such it can be used for many different things. Here I want to discuss the way analogy is used as the basis for making arguments--and this should be plenty useful to us, since much of what we do is make and evaluate arguments involving progressive issues.
When used as the basis of an argument, an analogy (1) identifies two items, (2) observes that they share a set of common properties, and (3) extrapolates to the conclusion that the two items share an additional property. Thus arguments by analogy always have the same formal structure:
1) Item 1 has properties {p1 - pn}. (for some number n)
2) Item 2 has properties {p1 - pn}.
3) Item 1 has the additional property q.
C) Therefore, Item 2 also has (likely has, should have) the additional property q.
Sweet!
Of course, very few people will actually provide the formal structure of the argument for us. We usually have to dig it out for ourselves. Just for fun, here is one of the very best arguments by analogy I've ever seen. It comes from Judith Jarvis Thomson's "A Defense of Abortion", one of the most famous philosophy papers of the last fifty years. The whole thing is well worth reading, but here is the most famous passage:
...now let me ask you to imagine this. You wake up in the morning and find yourself back to back in bed with an unconscious violinist. A famous unconscious violinist. He has been found to have a fatal kidney ailment, and the Society of Music Lovers has canvassed all the available medical records and found that you alone have the right blood type to help. They have therefore kidnapped you, and last night the violinist's circulatory system was plugged into yours, so that your kidneys can be used to extract poisons from his blood as well as your own. The director of the hospital now tells you, "Look, we're sorry the Society of Music Lovers did this to you--we would never have permitted it if we had known. But still, they did it, and the violinist is now plugged into you. To unplug you would be to kill him. But never mind, it's only for nine months. By then he will have recovered from his ailment, and can safely be unplugged from you." Is it morally incumbent on you to accede to this situation? No doubt it would be very nice of you if you did, a great kindness. But do you have to accede to it? What if it were not nine months, but nine years? Or longer still? What if the director of the hospital says. "Tough luck. I agree. but now you've got to stay in bed, with the violinist plugged into you, for the rest of your life. Because remember this. All persons have a right to life, and violinists are persons. Granted you have a right to decide what happens in and to your body, but a person's right to life outweighs your right to decide what happens in and to your body. So you cannot ever be unplugged from him." I imagine you would regard this as outrageous...
Now that is a kickass analogy!
Better still, it's a kickass argument by analogy. Take a look at the underlying structure:
(Note: For the purposes of this argument, Thomson granted the assumption that a fetus is a person. She wanted to show that abortion was morally justifiable even if she gave her opponents their most important assumption.)
1) Case 1 (The case of the violinist) has properties {'involving a victim of a violent crime', 'involving a situation for which the victim bears no moral responsibility', 'involving a situation which imposes significant potential future hardship on the victim'...}. (the list could go on some time!)
2) Case 2 (The case of pregnancy due to rape) has properties {'involving a victim of a violent crime', 'involving a situation for which the victim bears no moral responsibility', 'involving a situation which imposes significant potential future hardship on the victim'...}.
3) In the case of the violinist, there is the additional property 'the victim's having the right to disconnect the dependent person'.
C) Therefore, in the case of pregnancy due to rape, there is the additional property 'the victim's having the right to disconnect the dependent person'.
Very, very nice. I also want to note here that the properties being appealed to in this argument are all morally relevant properties. That's important.
As a point of contrast, let's try to make an argument by analogy out of the whole 'pregnancy due to rape'-'flat tire' comparison. (Psst--It's not gonna go so well.)
1) Case 1 (Flat tire!) has the properties {'being unplanned'...um 'being longer than 5 minutes in duration?'...um 'being entirely located within a mile of the earth's surface?'}
2) Case 2 (pregnancy due to rape) has the properties {'being unplanned', 'being longer than 5 minutes in duration', 'being entirely located within a mile of the earth's surface'}
3) In the case of the flat tire, there is the additional property 'the state's not assisting the victim'.
C) In the case of pregnancy due to rape, there should be the additional property 'the state's not assisting the victim'.
Argument fail.
Kansas state rep. Pete DeGraaf's analogy is a terrible basis for an argument by analogy. But note, it isn't because pregnancy due to rape and a flat tire can't be compared--they can be compared, and the two cases have lots of properties in common. (This isn't surprising--virtually any two things have lots of properties in common. For example--every object has the property 'being self-identical'! Similarity is very, very cheap.) The problem is that pregnancy due to rape and a flat tire don't have many relevant properties in common in the context of a debate about health insurance. Hence, argument fail.
As Thomson shows us, analogy can be a powerful basis for an argument; as DeGraaf shows us, analogy can be an argumentative pitfall for the stupid. A very nice way of separating the two is to formalize the argument and then remember the maxim: 'Similiarity is cheap; relevance is expensive.' By understanding the underlying structures of arguments, we learn to use good arguments more effectively and disarm bad arguments more efficiently.