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Why does a valid deductive argument preserve truth from premises to conclusion?
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Aristotle, marble portrait bust, Roman copy (2nd century bce) of a Greek original (c. 325 bce); in the Museo Nazionale Romano, Rome.
A valid deductive argument preserves truth from premises to conclusion because of its logical form or structure. In such an argument, the conclusion follows from the premises with strict necessity, meaning it is impossible for the premises to be true while the conclusion is false.1234
The preservation of truth is guaranteed by the following principles:
- Structural Necessity: Validity is independent of the specific subject matter or the actual truth of the statements. Instead, it relies on the argument's construction, where the relationship between the premises and the conclusion is such that any scenario in which the premises are true must also be a scenario in which the conclusion is true.254
- Inconsistency of Denial: A deductive argument is valid if asserting the premises while denying the conclusion would result in a logical contradiction.62
- Truth-Preserving Rules: Deductive logic employs rules of inference that are defined as truth-preserving; when these rules are applied correctly, they ensure that the conclusion cannot be false if the starting premises are true.4
Essentially, the conclusion of a valid deductive argument contains no information not already implicit in the premises.4