1 9 8c 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 yTogether with other facts, this was enough to identify the trainer, Straker, as
the person who stole Silver Blaze. In this case, it is the fact that something
didn’t occur that provides the basis for an inference to the best explanation.
Of course, Holmes’s inference is not absolutely airtight. It is possible that
Straker is innocent and Martians with hypnotic powers over dogs commit-
ted the crime. But that only goes to show that this inference is neither valid
nor deductive in our sense. It does not show anything wrong with Holmes’s
inference. Since his inference is inductive, it is enough for it to be strong.
Inferences to the best explanation are also defeasible. No matter how
strong such an inference might be, it can always be overturned by future ex-
perience. Holmes might later find traces of a sedative in the dog’s blood or
someone else might confess or provide Straker with an alibi. Alternatively,
Holmes (or you) might think up some better explanation. Still, unless and
until such new evidence or hypothesis comes along, we have adequate rea-
son to believe that Straker stole the horse, because that hypothesis provides
the best available explanation of the information that we have now. The
fact that future evidence or hypotheses always might defeat inferences to
the best explanation does not show that such inferences are all bad. If it did
show this, then science and everyday life would be in trouble, because so
much of science and our commonsense view of the world depends on infer-
ences to the best explanation.Which Explanation Is Best?
To assess such inferences, we still need some standards for determining which
explanation is the best. There is, unfortunately, no simple rule for deciding
this, but we can list some factors that go into the evaluation of an explanation.^3
First, the hypothesis should really explain the observations. A good explanation
makes sense out of that which it is intended to explain. In our original example,
the broken lock can be explained by a burglary but not by the hypothesis that a
friend came to see you (unless you have strange friends). Moreover, the hypoth-
esis needs to explain all of the relevant observations. The hypothesis of a mis-
taken police raid might explain the broken lock but not the missing valuables or
the lack of any note or police officers when you return home.
The explanation should also be deep. An explanation is not deep but shal-
low when the explanation itself needs to be explained. It does not help to
explain something that is obscure by citing something just as obscure. Why
did the police raid your house? Because they suspected you. That explana-
tion is shallow if it immediately leads to another question: Why did they
suspect you? Because they had the wrong address. If they did not have the
wrong address, then we would wonder why they suspected you. Without
an explanation of their suspicions, the police raid hypothesis could not ad-
equately explain even the broken lock.
Third, the explanation should be powerful. It is a mark of excellence in an
explanation that the same kind of explanation can be used successfully over
a wide range of cases. Many broken locks can be explained by burglaries.97364_ch09_ptg01_195-214.indd 198 15/11/13 10:47 AM
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