Reasoning

Deductive Reasoning

[Page 1814]The distinction between deductive and inductive reasoning, noted above, is the most fundamental and frequent one in the study of reasoning. Deductive reasoning yields conclusions that are necessary. Deduction is truth preserving. This is to say that in a valid deductive argument, when the premises are true, the conclusion must be true. It is the form of the argument that guarantees this. For example, given the truth of the statements, “If it is raining, then the street is wet,” and “It is raining,” it follows necessarily that the street is wet. Any argument having this form, and true premises, must have a true conclusion, regardless of what the content of the individual statements might be. Deduction is central to mathematical proof, hypothesis testing, problem-solving, and critical thinking.

There are three principal theories of the development of deductive reasoning: mental logic, metacognitive, and mental model theories. Mental logic theories maintain that when individuals engage in deductive reasoning, they are employing logical rules that are analogous to the rules that govern operations within a formal or symbolic logic. In this view, the types of rules available to the individual and the degree of coordination or organization across inferences become increasingly sophisticated from childhood through adolescence. According to metacognitive theories, the development of reasoning reflects increasing knowledge regarding the nature of inference—referred to as metalogical knowledge—and greater metacognitive awareness of, and control over, inference. Metalogical knowledge includes an understanding of key distinctions among types of inference, an appreciation that conclusions must be consistent with all possible states of affairs represented by the premises, and an understanding of logical consistency and necessity. Mental model theories contend that deductive reasoning involves the construction of a real-time representation of possible states of affairs denoted by the premises of an argument. Mental models are semantic in nature and are not based in logical rules. According to this theory, not all possibilities consistent with the premises of an argument are initially modeled, but additional possibilities can be fleshed out, as needed, through a more deliberative search.

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