The Language of Argument

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c 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 y

smaller percentage of objects that have properties P, Q, and R lack property

X, then the argument from these analogies will be strong.

If we are not sure which respects are important, we still might have some

idea of which respects might be important. Then we can try to cite objects

that are analogous in as many as possible of those respects. By increasing

the number of potentially relevant respects for which the analogy holds, we

can increase the likelihood that the important respects will be on our list.

That shows why arguments from analogy are usually stronger when they

cite more and closer analogies between the objects.

Another factor that affects the strength of an argument from analogy is

the presence of relevant disanalogies. Because arguments from analogy are de-

feasible, as we saw, a strong argument from analogy can become weak if we

add a premise that states an important disanalogy. Suppose my new car is

like my old cars in many ways, but there is one difference: The new car has

an electric motor, whereas the old cars were powered by gasoline. This one

difference is enough to weaken any argument to the conclusion that the new

car will be reliable. Of course, other disanalogies, such as a different color,

won’t matter to reliability; and it will often require background knowledge

to determine how important a disanalogy is.

We need to be careful here. Some disanalogies that are relevant do not un-

dermine an argument from analogy. If a new engine design was introduced

by top engineers to increase reliability, then this disanalogy might not under-

mine the argument from analogy. Differences that point to more reliability

rather than less might even make the argument from analogy stronger.

Other disanalogies can increase the strength of an argument from analogy

in a different way. If the same markings are found on very different kinds

of sacrificial knives, then the presence of those markings on the newly dis-

covered knife is even stronger evidence that this knife was also used in sac-

rifices. Differences among the cases cited only in the premises as analogies

(that is, B, C, D, and so on) can strengthen an argument from analogy.

Finally, the strength of an argument from analogy depends on its conclu-

sion. Analogies to other kinds of cars provide stronger evidence for a weak

conclusion (such as that the new model will probably be pretty reliable) and

weaker evidence for a strong conclusion (such as that the new model will

definitely be just as reliable as the old model). As with other forms of ar-

gument, an argument from analogy becomes stronger as its conclusion be-

comes weaker and vice versa.

These standards can be summarized by saying that an argument from

analogy is stronger when:

1. It cites more and closer analogies that are more important.


2. There are fewer or less important disanalogies between the object in
   the conclusion and the other objects.


3. The objects cited only in the premises are more diverse.


4. The conclusion is weaker.

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