Distinction Between Deductive and Inductive Reasoning

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Galotti (1994) points out several ways to explain the distinction between deductive and inductive reasoning:

1. One way to think about the difference is to say that deductive reasoning goes from the general to the specific or particular (e.g., "All college students like pizza. Terry is a college student. Therefore, Terry likes pizza."). Inductive reasoning goes from the specific to the general, (e.g., Brian is a college student. Brian lives in a dormitory. Therefore, all college students live in dormitories.").

2. Another way to describe the difference between the two types of reasoning is to say that in deductive reasoning, no new information is added; any conclusion drawn represents information that was already implicit in the premises. Inductive reasoning, on the other hand, can result in conclusions that contain new information.

3. A third and related way of talking about differences between deductive and inductive reasoning has to do with the claims that can be made for the kinds of conclusions drawn. Deductive reasoning, if performed correctly, results in conclusions that are said to have "deductive validity" (Skyrms, 1975). An argument is deductively valid if and only if it is impossible for the premises to be true and the conclusion (or conclusions) to be false. Deductive validity thus provides the reasoner with a nice guarantee: start with true premises, reason according to logical principles, and the conclusion you come to cannot be false. The argument about Terry and the pizza is a deductive argument: if it is true that all college students like pizza, and Terry is a college student, then we know that Terry likes Pizza.

However, many arguments are inductive reasoning rather than deductive, and in these cases, we cannot be certain of the conclusions; we can have only stronger or weaker beliefs in them. Take the argument about Brian living in the ...