Deductive+Logic

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Definition:
A type of reasoning, based on hypothetical premises, that requires predicting a specific outcome from a general principle. HGD, Chapter 9, p. 223

Based on hypothetical premises, starting with a general principle and then predicting some outcome or observation, is required. Deductive logical arguments are evaluated by their validity and soundness. It is possible to have a deductive argument that is logically "valid" but is not sound. An argument is //valid//if it is impossible for its premises to be true while its conclusion is false. In other words, the conclusion must be true if the premises, whatever they may be, are true. An argument can be valid even though the premisesare false. An argument is //sound// if it is valid and the premises are true. The example's first premise is false (there are people who eat steak that are not quarterbacks), but the conclusion must be true, so long as the premises are true (i.e. it is impossible for the premises to be true and the conclusion false). Therefore the argument is valid, but not sound. The theory of deductive reasoning known as categorical or term logic was developed by Aristotle, but was superseded by propositional (sentential) logic and predicate logic. Deductive reasoning can be contrasted with inductive reasoning. In cases of inductive reasoning, even though the premises are true and the argument is "valid", it is possible for the conclusion to be false (determined to be false with a counterexample or other means).

Examples:
Going from a theory to a hypothesis.

The following is an example of an argument that is valid, but not sound; a premise is false: or:
 * 1) Everyone who eats steak is a quarterback.
 * 2) John eats steak.
 * 3) Therefore, John is a quarterback.
 * 1) All quarterbacks eat steak.
 * 2) Tony is a quarterback.
 * 3) Therefore, Tony eats steak.

Analogies:
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Mnemonics:
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