2 0 4c hAp tEr 9 ■ I n f e r e n c e t o t h e B e s t E x p l a n a t i o n a n d f r o m A n a l o g yArguments from Analogy
Another very common kind of inductive argument moves from a premise
that two things are similar in some respects to a conclusion that they must
also be analogous in a further respect. Such arguments from analogy can be
found in many areas of everyday life. When we buy a new car, how can we
tell whether it is going to be reliable? Consumer Reports might help if it is an
old model; but if it is a brand-new model with no track record, then all we
can go on is its similarities to earlier models. Our reasoning then seems to be
that the new model is like the old model in various ways, and the old model
was reliable, so the new model is probably reliable, too.
The same form of argument is used in science. Here’s an example from
geology:
Meteorites composed predominantly of iron provide evidence that parts of other
bodies in the solar system, presumably similar in origin to Earth, were composed
of metallic iron. The evidence from meteorite compositions and origins lends
support to the conclusion that Earth’s core is metallic iron.^5
The argument here is that Earth is analogous to certain meteors in their origins,
and those meteors have a large percentage of iron, so the Earth as a whole prob-
ably contains about the same percentage of iron. Because a smaller amount of
iron is present in the Earth’s crust, the rest must lie in the Earth’s core.
Similarly, archaeologists might argue that a certain knife was used in rit-
ual sacrifices because it resembles other sacrificial knives in its size, shape,
materials, carvings, and so on. The analogy in this case is between the newly[During the Archean Era, which extended from about 3.8 to 2.5 million years before
the present,] the sun’s luminosity was perhaps 25% less than that of today.... This
faint young sun has led to a paradox. There is no evidence from the scant rock record
of the Archean that the planetary surface was frozen. However, if Earth had no at-
mosphere or an atmosphere of composition like that of today, the amount of radiant
energy received by Earth from the sun would not be enough to keep it from freez-
ing. The way out of this dilemma is to have an atmosphere present during the early
Archean that was different in composition that that of today.... For a variety of rea-
sons, it has been concluded, although still debated, that the most likely gases present
in greater abundance in the Archean atmosphere were carbon dioxide, water vapor
(the most important greenhouse gas) and perhaps methane. The presence of these
greenhouse gases warmed the atmosphere and planetary surface and prevented the
early Archean Earth from being frozen.^4- Find three more inferences to the best explanation in articles about science
in a newspaper, magazine, or professional journal. This should be easy
because scientists often use this form of argument. Put those inferences in
standard form, and then evaluate them using the tests discussed above.
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