Wednesday, 17 July 2013

What is "Deductive Reasoning"?

By Joshua Schechter

Deductive reasoning is the kind of reasoning in which, roughly, the truth of the input propositions (the premises) logically guarantees the truth of the output proposition (the conclusion), provided that no mistake has been made in the reasoning. The premises may be propositions that the reasoner believes or assumptions that the reasoner is exploring. Deductive reasoning contrasts with inductive reasoning, the kind of reasoning in which the truth of the premises need not guarantee the truth of the conclusion.

For example, a reasoner who infers from the beliefs
(i) If the room is dark then either the light switch is turned off or the bulb has burned out;
(ii) The room is dark;
(iii) The light switch is not turned off;
to the conclusion
(iv) The bulb has burned out;
is reasoning deductively. If the three premises are true, the conclusion is guaranteed to be true.

By contrast, a reasoner who infers from the belief
(i) All swans that have been observed are white;
to the conclusion
(ii) All swans are white;
is reasoning inductively. The premise provides evidential support for the conclusion, but does not guarantee its truth. It is compatible with the premise that there is an unobserved black swan.

Deductive reasoning has been intensively studied in cognitive science, psychology, and philosophy. There are many important debates concerning the nature of deductive reasoning. This entry surveys three topics – the relationship between deductive reasoning and logic, the main psychological models of deductive reasoning, and the epistemology of deductive reasoning.

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