Deductive reasoning is (very
roughly) the kind of reasoning in which the premises logically entail the
conclusion, at least assuming that no mistake has been made in the reasoning.
The premises of a deductive argument may be propositions that the thinker believes
or propositions that the thinker temporarily assumes to be true in order to
explore their consequences. Deductive reasoning contrasts with inductive (or ampliative ) reasoning, the kind of reasoning in which the truth of the premises
does not guarantee the truth of the conclusion.
One important question concerning deductive
reasoning is whether we do, in fact, engage in anything that could reasonably be
called “deductive reasoning”. Some philosophers and psychologists have denied
that there is any such thing, though the consensus in both psychology and
philosophy seems to be that there is a distinctive kind of deductive reasoning.
Within cognitive science, an important question concerns the nature of deductive
reasoning – does it depend on applying rules to mental representations that
resemble the sentences of natural language or does it involve reasoning with
diagrammic models? Within philosophy, there are important questions concerning
the epistemology of deductive reasoning. Which deductive rules are thinkers
justified in employing? What makes it the case that certain rules of inference
preserve justification (or knowledge) rather than others?