Archaeological Example
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. De- spite 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 discov- ered 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 argu- ment from analogy does not provide much, if any, support for its conclusion.
In addition, the cited similarities must be relevant. Suppose someone ar- gues 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 proba- bly 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 reliability depends on the drive train and the engine rather than on the color or the sunroof.
