2 0 7A r g u m e n t s f r o m A n a l o g yAre Analogies Explanations?
After learning about arguments from analogy, it is natural to wonder how
they are related to inferences to the best explanation. Although this is some-
times disputed, it seems to us that arguments from analogy are often—if not
always—implicit and incomplete inferences to the best explanation. As we
pointed out, analogies don’t support any conclusion unless they are rele-
vant, and whether they are relevant depends on how they fit into explana-
tions. The color of a car is irrelevant to its reliability, because color plays no
role in explaining its reliability. What explains its reliability is its drive train
design, materials, care in manufacturing, and so on. That is why analogies in
those respects can support a conclusion about reliability. Similarly, the mark-
ings on an artifact are relevant to whether it is a sacrificial knife if the best
explanation of why it has those markings is that it was used in sacrifices.
What makes that explanation best is that it also explains similar markings
on other sacrificial knives. Thus, such arguments from analogy can be seen
as involving an inference to the best explanation of why objects B, C, D, and
so on have property X followed by an application of that explanation to the
newly discovered object A.
Sometimes the explanation runs in the other direction. Whereas the con-
clusion about the knife’s use (X) is supposed to explain its shared markings
(P, Q, R), sometimes it is the shared features (P, Q, R) that are supposed to
explain the feature claimed in the conclusion (X). Here is a classic example:We may observe a very great [similarity] between this earth which we inhabit,
and the other planets, Saturn, Jupiter, Mars, Venus, and Mercury. They all revolve
around the sun, as the earth does, although at different distances and in different
periods. They borrow all their light from the sun, as the earth does. Several of
them are known to revolve around their axis like the earth, and, by that means,
must have a like succession of day and night. Some of them have moons that
serve to give them light in the absence of the sun, as our moon does to us. They
are all, in their motions, subject to the same law of gravitation, as the earth is.
From all this similarity it is not unreasonable to think that those planets may, like
our earth, be the habitation of various orders of living creatures. There is some
probability in this conclusion from analogy.^6The argument here seems to be that some other planet probably supports
life, because Earth does and other planets are similar to Earth in revolving
around the sun and around an axis, getting light from the sun, and so on.
What makes certain analogies relevant is not, of course, that the motion of
Earth is explained by the presence of life here. Rather, certain features of
Earth explain why Earth is habitable. The argument suggests that the best
explanation of why there is life on our planet is that certain conditions make
life possible. That generalization can then be used to support the conclusion
that other planets with the same conditions probably support life as well.
In one way or another, many (or maybe even all) arguments from anal-
ogy can be seen as inferences to the best explanation. But they are usually97364_ch09_ptg01_195-214.indd 207 15/11/13 10:47 AM
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