The Language of Argument

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A r g u m e n t s f r o m A n a l o g y

discovered knife and the other knives. This analogy is supposed to support

a conclusion about the function of the newly discovered knife.

Although such arguments from analogy have diverse contents, they share

a common form that can be represented like this:

(1) Object A has properties P, Q, R, and so on.

(2) Objects B, C, D, and so on also have properties P, Q, R, and so on.

(3) Objects B, C, D, and so on have property X.

∴(4) Object A probably also has property X.

In the archaeological example, object A is the newly discovered knife, and

objects B, C, D, and so on are previously discovered knives that are known

to have been used in sacrifices. Properties P, Q, R, and so on are the size,

shape, materials, and carvings that make A analogous to B, C, D, and so on.

X is the property of being used as a sacrificial knife. Premise 3 says that the

previously discovered artifacts have this property. The conclusion, on line 4,

says that the newly discovered artifact probably also has this property.

Since arguments from analogy are inductive, they normally aren’t valid.

It is possible that, even though this knife is analogous to other sacrificial

knives, this knife was used to shave the king or just to cut bread. These ar-

guments are also defeasible. The argument about knives obviously loses all

of its strength if we find “Made in China” printed on the newly discovered

knife. Still, none of this shows that arguments from analogy are no good.

Despite being invalid and defeasible, some arguments from analogy can still

provide reasons—even strong reasons—for their conclusions.

How can we tell whether an argument from analogy is strong or weak?

One obvious requirement is that the premises must be true. If the previously

discovered knives were not really used in sacrifices, or if they do not really

have the same carvings on their handles as the newly discovered knife, then

this argument from analogy does not provide much, if any, support for its

conclusion.

In addition, the cited similarities must be relevant. Suppose someone

argues that his old car was red with a black interior and had four doors

and a sunroof, and his new car also has these properties, so his new car is

probably going to be as reliable as his old car. This argument is very weak

because the cited similarities are obviously irrelevant to reliability. Such

assessments of relevance depend on background beliefs, such as that reli-

ability depends on the drive train and the engine rather than on the color

or the sunroof.

The similarities must also be important. Similarities are usually more im-

portant the more specific they are. Lots of cars with four tires and a motor

are reliable, but this is not enough to infer that, because this particular car

also has four tires and a motor, it will be reliable, too. The reason is obvious:

There are also lots of unreliable cars with four tires and a motor. In general,

if many objects have properties P, Q, and R, and many of those lack prop-

erty X, then arguments from these analogies will be weak. In contrast, if a

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