Another very common kind of inductive argument is an argument from analogy.
We'll see that these arguments from analogy are very closely related to
inferences to the best explanation. But first, we got to ask, what's an
analogy? Analogy is basically a comparison between
two things. It points out similarities between those
two things. And analogies are given all the time.
For example, a poet might say, her eyes were like emeralds.
Well, in what way were they like emeralds?
They might have had the same color, they might have shimmered like emeralds, they
might have been valuable like emeralds. And the analogy, her eyes were like
emeralds, doesn't really tell you exactly which respect her eyes resembled
emeralds, but that's part of the point.
When you're writing poetry, you want to stimulate creative comparisons and
analogies of, so that readers of the poem can think about it in their own way and
the same thing holds for other analogies in other areas.
But because it's not very specific, some people think that analogies are just no
good at all in arguments. Actually though, we use analogies and
arguments all the time. Here's an example from public policy.
They built a transportation system in the city of Houston, Texas.
it worked pretty well. And then, the planners in the city of
Phoenix, Arizona were wondering what kind of public transportation system to build
there and they reasoned like this. They said, Phoenix is a lot like Houston
in many ways, large population, hot during the summer,
many, many people and large area. So, they said, Phoenix resembles Houston
in a lot of ways. This type of transportation system worked in Houston,
so it'll probably work well in Phoenix also.
Now, what about law? Lot's of legal decisions are based on
analogies too, because common law systems at least follow precedent.
When judges decide a case one way in one time, at one point, then later on other
judges are suppose to make similar decisions.
So you can say for example, the Supreme Court declared that segregated public
high schools are unconstitutional in the United States.
Colleges are a lot like high schools, so, segregated public colleges are also
unconstitutional in the United States. And then, med schools are a lot like
colleges, so, segregated public medical schools are
also unconstitutional in the United States.
An that's the way the legal system evolves by drawing analogies among the
different cases that come up within that jurisdiction.
This form of argument in law might seem to be a real problem, because you don't
say exactly what the similarities are. But actually, it's very useful, because
it is predictable if you know that segregated high schools have been
declared unconstitutional, you pretty know, much know that judges are going to
find colleges unconstitutional too if they're segregated.
and it also gives flexibility, so that judges can see when they're going
too far, they say, well, that precedent is different and they distinguish the
precedents. So by resting legal reasoning on
arguments from analogy, they gain both predictability and also flexibility in
the legal system. So arguments from analogy can be pretty
useful. Fine,
but policies and laws are all about norms and values.
What about science and hard facts? Well, sciences use analogies, too.
For example, scientists at one point didn't know what was at the center of the
earth, but they found a bunch of meteors and
meteorites that had a high iron content, much higher than the content of iron in
the crust of the earth. So, they reasoned the earth must be like
these other meteors and meteorites, so it must have a similar amount of iron
in it, because they were produced in the same way in the history of the universe.
But that means there must be a similar amount of iron in the Earth.
If it's not in the crust, where could it be?
It must be down in the core. So they figured, probably,
the core of the earth has a lot of iron in it.
That's just one example, but scientists actually use analogies a lot, and if you
don't believe me, go read some psychological studies of scientific
reasoning. But we're going to focus on an example
from art history. Just imagine that you're going through
the attic and you find an old painting. It looks a lot like a painting by the
famous impressionist Cezanne, and if it is by Cezanne, it's worth an
awful lot. But you got to figure out whether this
painting is by Cezanne. How do you figure it out, because Cezanne
didn't sign it? He didn't sign a lot of his paintings.
Well, what you do is you look at other Cezanne
paintings and try to figure out whether they're similar. And if you're not an
expert, probably better check with an expert and have them do it, but they're
going to do the same thing. They're going to compare this painting to
a lot of other paintings that we know are by Cezanne, and then you can reason like
this, this painting has a certain kind of
brushwork and coloring and so on, and so on, subject matter, whatever.
other paintings by Cezanne have very similar brushwork and color patterns and
topic, and so on. Those other paintings are definitely by
Cezanne, we know that, therefore this painting is probably by Cezanne as well.
Now, that's an argument from analogy. This argument from analogy shares a
certain form with the other arguments from analogy that we discussed before.
And we can pick out that form by substituting letters for the English
words in the English argument. For example, we can substitute the letter
a for the subject, that is the topic of the argument,
the painting that we don't know whether it's a Cezanne or not.
And we can substitute the letters B, C, and D for the similar objects that are
also Cezanne paintings. And we can substitute the letters P, Q,
and R for the similarities between the paintings that we know are by Cezanne and
the one that we're not sure of. And then we can substitute the letter X
for that particular property of being by Cezanne.
And when you substitute all those letters for the English words, then the argument
simply says that object A has properties P, Q, and R and objects B, C, and D also
have those properties P, Q, and R and B, C, and D also have the property X.
So the subject, object A, probably also has the property X,
namely, this painting is by Cezanne. Probably.
Of course, since this argument only tries to show that the conclusion is probably
true, it's an inductive argument. It's not valid, it's possible for the
premises to be true and the conclusion false, namely, this painting might
resemble all those other paintings in those respects and yet it's not by
Cezanne. Secondly, the argument is defeasable.
You can get some additional information that makes it really look like a bad
argument. For example, you could turn the painting
over and on the back you find the signature of a different artist like
Sirah and then you realize this isn't by Cezanne at all.
But nonetheless, the argument can be strong.
It can always be stronger, because there can be more similarities and more
important similarities. But, it can be a strong argument and a
good argument, because it's inducted, so it doesn't event try or pretend to be
valid. How can we tell when an argument for
analogy really does give us a strong reason to believe the conclusion?
What are the standards by which we measure how strong the argument is and
how strong the reasons are? Well, one of them should be obvious.
You know, of course, the premises have to be true and justified like in any
argument. A standard that's relevant here is that
when there are more important analogies, then it provides a stronger reason,
because some analogies are just totally unimportant.
The painting is square, other paintings by Cezanne are square,
therefore, this is by Cezanne. Well, that's ridiculous, right?
Because lots of painters use square canvases.
For something that's important, that's going to be specific to Cezanne and very,
very idiosyncratic is going to be more important for this type of argument.
Secondly, when there are more analogies, because we don't know exactly which one
is the one that's important. That's the point of an argument from
analogy. You draw the analogy without knowing
exactly which respect is the crucial one, so the more analogies that you have, the
more likely you're going to hit on the ones that are crucial.
So, if it's not just brushwork, it's also the type of paint that was used.
It's also the color scheme that was used. It's also the geometric shapes.
It's also the subject matter. It's a particular mountain that's close
by where Cezanne lived and he painted a lot of that mountain, and on, and on, and
on. The more analogies, the more likely that
some of them are going to be the crucial ones.
And therefore, the stronger the argument is and the stronger reason it gives you
to believe that this particular painting is by Cezanne.
But of course, there are always going to be some disanalogies as well, because
Cezanne didn't paint the same thing over and over again exactly the way he did the
first time. The fewer disanalogies, the stronger the
argument. There'll always be some, or there
wouldn't be much of an argument, they'd be exactly the same thing.
But the fewer disanalogies and the less important those disanalogies are, then
the stronger the argument is going to be. Next, the objects that you're comparing,
because they're similar in various respects, that is, the other paintings
that we know are by Cezanne. If they're quite diverse, then, that
means that you have similarities among a diverse group that all share these
particular properties. And that means that Cezanne continued to
use those features throughout all the different types of paintings that he did
and that means that it's going to be a stronger reason to believe that this
painting is by Cezanne. Finally, the conclusion is weaker.
You could say, therefore, this painting is definitely by Cezanne,
it couldn't be anybody else. Well, that's kind of crazy, right?
But if you say, it's probably by Cezanne, it has some chance it's by Cezanne, maybe
you want to check it further. Then, you're weakening the conclusion and
that can make the argument stronger. So in all of these different ways, we can
assess how strong the argument from analogy is by looking at the respects in
which the objects are analogous, the diversity among the objects that are
analogous, the strength of the conclusion, and so on and so on.
And that's how we assess an argument from analogy for strength.
I want to close with one more example that raises interesting questions about
the relationship between arguments from analogy and inferences to the best
explanation. It concerns the pressing issue of whether
Neanderthals were cannibals. Now it's not a pressing issue for most
people, but it is a very pressing issue for people who study Neanderthals.
And so it's quite a breakthrough when they found some bones in a cave that they
knew was inhabited by Neanderthals. In that cave, next to what looked like a
fire pit, there were bones of deer with markings of a certain sort that looked
like they had been cutting the meat off the bone.
And they also found bones of humans in that cave where they had been cutting,
where they had similar markings. And they argued,
since the bones have similar markings and these bones,
the bones of the deer, were probably cut up for food.
Well, the human bones where probably also cut up for food.
So, they reached the conclusion that at least, sometimes, Neanderthals ate
humans. What's interesting is that there are two
ways to reconstruct this argument. First, you can reconstruct it as an
argument from analogy. The bones of the humans were found in
this location with these kinds of markings.
The bones of deer were also found in this location with these kinds of markings,
but the deer were cut up for food, therefore, the humans were probably also
cut up for food. Now, that sounds like an argument from
analogy when you think about it that way. But you can also reconstruct the argument
as an inference to the best explanation. The bones of the humans had these
markings on them and we're in this location,
how do you explain that? The best explanation of why they have
these particular kinds of markings is that they were cut up for food,
therefore, the humans were probably cut up for food as well.
Now notice, that both reconstructions of the argument make the argument look okay.
And so, it's not clear, which tells you the real structure of the argument that
the author was trying to give. And that means that arguments from
analogy and inferences to the best explanation are actually very closely
related, and sometimes, you can take an argument
and reconstruct it either way. It's not going to affect very much how
strong the argument is, but it might affect how you see the argument working.
And the big difference is that, when you do an argument from analogy.
You don't have to specify exactly which respect is important,
so you can put out lots of analogies and hope that you hit the one that really
matters. Whereas when you're doing an inference to
the best explanation, then you have to pick out the specific
property that gives you the explanation of the phenomenon that you observed.
So it forces you to get a little bit more specific than with an argument from
analogy. But otherwise, these two arguments are
clearly very closely related and they're basically two different ways to argue for
similar conclusions. For example, in this case, with the
argument from analogy, you don't know whether it's the location next to the
fire or the types of markings or maybe there are several different types of
markings and they're all the same, but you don't know which ones are the ones
that indicate how it was killed and which markings indicate how it was cut up to be
eaten. And an argument from analogy can leave
all that vague and just think that probably one of those similarities
justifies the conclusion that the humans were cut up to be eaten.
But if you're going to give an inference to the best explanation, then you're
saying that these markings, for example, diagonal markings on leg bones, might
suggest that they cut that up in a certain way, because that's how they
prepared the deer meat and they were used to preparing deer meat that way.
So they used similar cutting techniques when they were preparing human meat and
you've got an explanatory story that's much more specific than a mere analogy,
but it also commits you to a lot. So it might be questionable in various
ways. So when you look at an argument like
this, you've gotta decide which way to reconstruct the argument.
It's an argument from analogy, or it's an inference to the best explanation.
And the general rule is one that we saw long ago, in early weeks of this course,
if you really want to understand an argument, you want to understand your
opponents or you want to have a better argument for yourself, then you try to
make the argument look as good as possible.
So when you face a particular example like the Neanderthal example, you have to
decide is this argument be, going to be better if I reconstruct it as an argument
from analogy or is it going to be better if I reconstruct it as an inference to
the best explanation? And the best reconstruction is going to
be the one that makes the argument look best.