2 0 1I n f e r e n c e s t o t h e B e s t E x p l a n a t i o nContext can also affect the rankings of various factors. Many explanations,
for example, depend on universal premises. In such cases, compatibility with
observation is usually the primary test. The universal principle should not
be refuted by counterexamples (see Chapter 17). But sometimes explanatory
power will take precedence: If a principle has strong explanatory power, we
may accept it even in the face of clear disconfirming evidence. We do not
give up good explanations lightly—nor should we. One reason is that we do
not test single propositions in isolation from other propositions in our system
of beliefs. When faced with counterevidence to our beliefs, we often have a
choice between what to give up and what to continue to hold on to. A simple
example will illustrate this. Suppose that we believe the following things:
(1) Either John or Joan committed the crime.
(2) Whoever committed the crime must have had a motive for doing so.
(3) Joan had no motive to commit the crime.
From these three premises we can validly infer that John committed the crime.
Suppose, however, that we discover that John could not have committed the
crime. (Three bishops and two judges swear that John was somewhere else at
the time.) Now, from the fact that John did not commit the crime, we could
not immediately conclude that Joan committed it, for that would lead to an
inconsistency. If she committed the crime, then, according to premise 3, she
would have committed a motiveless crime, but that conflicts with premise 2,
which says that motiveless crimes do not occur. So the discovery that John
did not commit the crime entails that at least one of the premises in the argu-
ment must be abandoned, but it does not tell us which one or which ones.
This same phenomenon occurs when we are dealing with counterevi-
dence to a complex system of beliefs. Counterevidence shows that there must
be something wrong somewhere in the system, but it does not show exactly
where the problem lies. One possibility is that the supposed counterevidence is
itself in error. Imagine that a student carries out an experiment and gets the re-
sult that one of the fundamental laws of physics is false. This will not shake the
scientific community even a little, for the best explanation of the student’s re-
sult is that she messed things up. Given well-established principles, she could
not have gotten the result she did if she had run the experiment correctly. Of
course, if a great many reputable scientists find difficulties with a supposed
law, then the situation is different. The hypothesis that all of these scientists,
like the student, simply messed up is itself highly unlikely. But it is surpris-
ing how much contrary evidence will be tolerated when dealing with a strong
explanatory theory. Scientists often continue to employ a theory in the face of
counterevidence. Sometimes this perpetuates errors. For years, instruments re-
ported that the levels of ozone above Antarctica were lower than before, but
scientists attributed these measurements to bad equipment, until finally they
announced an ozone hole there. Still, there is often good reason to hold on to a
useful theory despite counterevidence, as long as its defects do not make seri-
ous trouble—that is, give bad results in areas that count. Good judgment is
required to determine when it is finally time to shift to a different explanation.97364_ch09_ptg01_195-214.indd 201 15/11/13 10:47 AM
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