Thinking and Deciding

96 LOGIC
Formal syllogisms illustrate a kind of warrant. For example, consider a syllogism
much like those discussed earlier in this chapter:
Anne is one of Jack’s sisters.
All of Jack’s sisters have red hair.
So, Anne has red hair.
Here, the first premise (“Anne is one ...”) is the datum; the second premise (“All
of Jack’s sisters ...”) functions as the warrant; and the conclusion (“So ...”) isthe
claim. Note that no qualification is necessary here. (There are other kinds of warrants,
such as those used in mathematics, that do not require qualification.) Moreover,
the claim follows by virtue of the meaning of the statements. We can paraphrase
the second premise as “Any sister of Jack’s can be taken to have red hair.” In such
cases, we can replace the “So ...” with “In other words, ...” The claim is practically
a restatement of the warrant, with the particular term (“Anne”) substituted
for the general term (“All of Jack’s sisters”). Toulmin calls this sort of syllogism
analytic, meaning, roughly, that the claim follows from an analysis of the premises. 7
Note that even the syllogism just given can have different meanings, depending
on the backing of the warrant. The backing is not stated in formal syllogisms, but
it does matter. For example, the original syllogism would be valid if the backing
were “Each of Jack’s sisters has been checked individually and found to have red
hair.” Suppose, however, the backing were “All of Jack’s sisters have previously
been observed to have red hair.” Now the syllogism is not analytic, and a qualifier is
needed:
Anne is one of Jack’s sisters.
All of Jack’s sisters have red hair (a statement backed by the fact that
they have previously been observed to have red hair).
So, presumably, Anne has red hair (unless she dyed her hair, her hair
turned white, etc).
The rebuttal, the reason for the qualifier, is the clause beginning with “unless.”
In sum, we might say that formal logic is concerned with some of the arguments
that require no qualifiers. Toulmin expands the design of logic by admitting other
kinds of warrants, and he suggests a more detailed study of the various kinds of
warrants used in different fields of knowledge. He wants to know what acceptable
warrants have in common across fields and how they differ from field to field.
Now let us see if we can restate Toulmin’s design in terms of the search-inference
theory. Toulmin’s datum clearly corresponds to a piece of evidence. When possibilities
are suggested by evidence, we can say that a claim corresponds to a possibility. If
no qualifier is required, the evidence is sufficient to make that possibility acceptable,
and thinking can stop right there. If qualifiers are required, other evidence and other
7The term “analytic” is usually contrasted with “synthetic,” but Toulmin prefers the term “substantive”
as a contrast.