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