1 9 9I 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 nExplanatory range is especially important in science. One of the main rea-
sons why Einstein’s theory of relativity replaced Newtonian physics is that
Einstein could explain a wider range of phenomena, including very small
particles at very high speeds.
Explanations go too far, however, when they could explain any possible
event. Consider the hypothesis that each particle of matter has its own in-
dividual spirit that makes it do exactly what it does. This hypothesis might
seem to explain some phenomena that even Einstein’s theory cannot explain.
But the spirit hypothesis really explains nothing, because it does not explain
why any particle behaves one way as opposed to another. Either behavior is
compatible with the hypothesis, so neither is explained. To succeed, there-
fore, explanations need to be incompatible with some possible outcome. In
short, they need to be falsifiable. (See Chapter 16 on self-sealers.)
Moreover, explanations should be modest in the sense that they should not
claim too much—indeed, any more than is needed to explain the observa-
tions. When you find your lock broken and valuables gone, you should not
jump to the conclusion that there is a conspiracy against you or that gangs
have taken over your neighborhood. Without further information, there is no
need to specify that there was more than one burglar in order to explain what
you see. There is also no need to hypothesize that there was only one burglar.
For this reason, the most modest explanation would not specify any number
of burglars, so no inference to the best explanation could justify any claim
about the number of burglars, at least until more evidence comes along.
Modesty is related to simplicity. One kind of simplicity is captured by the
celebrated principle known as Occam’s razor, which tells us not to multiply
entities beyond necessity. Physicists, for example, should not postulate new
kinds of subatomic particles or forces unless there is no other way to ex-
plain their experimental results. Similar standards apply in everyday life.
We should not believe in ghosts unless they really are necessary to explain
the noises in our attic or some other phenomenon. Simplicity is not always
a matter of new kinds of entities. In comparison with earlier views, the the-
ory that gases are composed of particles too small to see was simpler inso-
far as the particle theory allowed gas laws to be explained by the standard
physical principles governing the motions of larger particles without having
to add any new laws. Simplicity is a mark of excellence in an explanation
partly because simple explanations are easier to understand and apply, but
considerations of plausibility and aesthetics are also at work in judgments of
which explanation is simplest.
The tests of modesty and simplicity might seem to be in tension with the
test of power. This tension can be resolved only by finding the right balance.
The best explanation will not claim any more than is necessary (so it will be
modest), but it will claim enough to cover a wide range of phenomena (so it
will be powerful). This is tricky, but the best explanations succeed in recon-
ciling and incorporating these conflicting virtues as much as possible.
Finally, an explanation should be conservative. Explanations are bet-
ter when they force us to give up fewer well-established beliefs. We have97364_ch09_ptg01_195-214.indd 199 15/11/13 10:47 AM
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