Evaluation of Arguments by Analogy:
Finding the Middle Term

In some cases, it is easy to identify the middle term, especially if the argument explicitly mentions what A and B have in common.

In many cases, however, the common property is not mentioned explicitly; there may be more than one common property, and it may not be clear which ones are relevant to the conclusion.

A useful technique is to construct a table of similarities and differences. In outline, the table would look like the following:

A
B
S1
S1
Similarities
S2
S2
S3
S3
Differences
D1
D1
D2
D2
____
P P

The two columns represent the properties of A and B. Because the conclusion of the argument is the claim that B is P, we put P at the bottom and draw a line above it in the B column to indicate that it is supposed to follow from information available in the rest of the table.

S1, S2, S3, and so forth--there could be any number--are similarities between A and B, properties that they share and that are candidates for the role of the middle term.

To decide which of them is the middle term, we ask which of them seem connected to P. If they are all relevant, then the middle term is a combination: S1 + S2 + S3 . . . Usually, however, we can throw some of the similarities out as irrelevant to the analogy.

It's a good idea to include any differences (D1, D2, . . .) as well, because we must consider these when we evaluate the inductive element in the argument.

Once we have selected the most plausible middle term, and analyzed the argument accordingly, we need to evaluate the inductive step.

We are supporting a claim about B on the basis of its similarity to A, so A is the only instance available to support the generalization. Also, we have seen that a single instance usually does not provide very much evidence for a general proposition.

In this respect, an argument by analogy is a kind of logical shortcut, and it is a relatively weak mode of argument. Nevertheless, such arguments vary a great deal among themselves in their degree of strength, and we can assess their strength by applying our rules for evaluating generalizations.


Finding the middle term | Rules for evaluation

Evaluation of Arguments by Analogy

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