Thinking and Deciding

94 LOGIC
be made with such absolute certainty as the conclusion of a logical syllogism (even
when we can be sure of the premises). For example, it does not follow logically,
from the fact that smokers are much more likely to get lung cancer than nonsmokers,
that smoking causes lung cancer, but one would be a fool to insist on a logically
conclusive proof before concluding that smoking is dangerous to your health. If logic
could be made more relevant to everyday reasoning, it would be far more useful.
Toulmin (1958) tries to account for the structure of all arguments, not just those
treated by formal logic. He suggests that arguments have a basic structure (illustrated
in the following diagram) that includes four basic elements: datum, claim, warrant,
and backing.
Datum ✲ Claim
Warrant
Backing
For example, consider the argument “Harry was born in Bermuda. So, presumably,
Harry is a British subject.” The datum (fact) here is that Harry was born in Bermuda.
The claim is that he is a British subject. The warrant here is not explicitly stated, but
if it were, it would be, “A man born in Bermuda is usually a British subject.” The
warrant is the reason why the claim follows from the datum. Just as the claim can
be introduced with “so” or “therefore,” the warrant can be introduced with “since”
or “because.” Harry’s birth bears on his citizenship because people born in Bermuda
are usually British subjects. Finally, the backing is the justification for the warrant,
the reason we accept it. Here, the backing consists of the various laws pertaining to
British citizenship.
Notice that the word “presumably” had to be inserted in the argument. Toulmin
calls this a qualifier. The qualifier indicates that there are conditions under which the
conclusion does not follow. Qualifiers are necessary with certain kinds of warrants.
In this case, there are various reasons why Harry might not be a British subject, even
though he was born in Bermuda.
Toulmin adds a final element, the rebuttal (of the claim), which consists of the
reasons why the claim might not hold. The rebuttal can be introduced with the word
“unless.” In the case of Harry, the rebuttal would be, “unless both of Harry’s parents
were aliens, or Harry became an American citizen, or ....” The rebuttal gives the
reasons why the qualifier is necessary. The rebuttal is like a warrant, but it argues
against the claim rather than in favor of it. Just as the warrant justifies the claim, the
rebuttal justifies the qualifier. Thus, the complete structure of an argument can be
diagrammed as follows: