In logic, a type of inference or argument that purports to be valid, where a valid argument is one whose conclusion must be true if its premises are true (see validity). Deduction is thus distinguished from induction, where there is no such presumption. Valid deductive arguments may have false premises, as demonstrated by the example: “All men are mortal; Cleopatra is a man; therefore, Cleopatra is mortal.” Invalid deductive arguments sometimes embody formal fallacies (i.e., errors of reasoning based on the structure of the propositions in the argument); an example is “affirming the consequent”: “If A then B; B; therefore, A” (see fallacy; formal and informal).
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The D-N model is taught implicitly in schools, and approximates our pre-theoretical conception of science, which many non-experts hold. It was initially formalized by Carl Hempel and Paul Oppenheim in their article Studies in the Logic of Explanation (1948). A sketch of it can be found in Karl Popper's Logic of Scientific Discovery (1959).
In the D-N model, at least one of the statements si must be a "law-like" statement--a problematic concept, but initially thought to be captured by universal affirmatives, i.e., statements of the form "all X are Y." The explanans must be appropriately testable or observable--they must have "empirical content." If the premises are all true and if the argument is deductively valid, then the following constitutes a correct deductive-nomological explanation of p:
s1. . . sn, therefore, p
As a very simple illustration, consider the following: we observe that a piece of chalk falls when released. Why does the chalk fall? A D-N explanation might look like this (without attending to all subtleties in the precisely correct statement of the premises and conclusion):
The model is positivist in tone and implication, devised as a prescriptive form for scientific explanations. Due to the way that the model eschews any account of causality, scientific modelling, or simplification--and the general rejection of logical positivism--it is no longer accepted as dogma.
The Effects of Deductive and Guided Inductive Instructional Approaches on the Learning of Grammar in the Elementary Foreign Language College Classroom
Jul 01, 2007; Abstract: This study investigates the effectiveness of deductive and guided inductive approaches for teaching grammar in...