Thinking and Deciding

CONCLUSION 97
possibilities can be considered. When possibilities are already present, evidence may
strengthen them or weaken them through the same kind of warrant.
Toulmin thus calls attention to the need for warrant in informal reasoning. When
somebody says, “The weather forecast said it will rain, so, presumably, it will snow,”
the statement sounds odd because it is difficult to imagine a warrant that connects
the datum (the forecast) and the claim (snow). Likewise, in thinking, the effect of a
piece of evidence on a possibility depends on a warrant.
The search-inference framework goes beyond Toulmin in that it describes thinking
as involving (typically) more than one possibility. Toulmin can incorporate other
possibilities only as rebuttals. If we are trying to determine Harry’s citizenship, for
example, evidence that he was born in Bermuda does support the possibility that he
is British. A thorough thought process would, however, consider other possibilities,
such as United States or Canadian citizenship, and other evidence, such as the fact
that he has a Texas accent. Rather than making rebuttals part of the basic argument,
we might think of alternative possibilities as the general rebuttal, “unless some other
possibility is better.”
Moreover, the view of thinking as selecting among possibilities for certain purposes
or goals calls attention to the fact that the usefulness of warrants depends on
goals. The datum that a Cadillac is heavier than most cars does not imply that I
ought to buy one if my only goal is fuel economy, but it does if my only goal is
safety. Although Toulmin’s approach is sensitive to this role of goals, it is limited in
being concerned only with inference, not with search.
Conclusion
Formal logic, by its very nature, is not a complete theory of thinking. Because logic
covers only inference, it cannot help us to understand errors that result from insufficient
search. When we treat logic problems as examples of problems in general,
however, they do serve as good illustrations of the effects of certain types of poor
thinking: the failure to consider alternatives to an initial conclusion or model and the
failure to seek counterevidence.
The theories of everyday reasoning that best describe human inferences are those
that are most consistent with the search-inference framework. These theories allow
that good inferences do not have to be certain and can admit qualification. Once
the idea of uncertainty is admitted, the “goodness” of an argument depends on the
relative weight of arguments for and against a possible conclusion and on the relative
support that the evidence gives to this possibility as opposed to others. The theories
of inference discussed later in this book all take this view.