Method
Deductive Reasoning
Classification
- Method family
- argumentative
- Domains
- Philosophy of Religion
Overview
Deductive reasoning moves from accepted premises to a conclusion they guarantee, so that accepting the premises while denying the conclusion is inconsistent.
Purpose. The method transmits whatever support the premises have to the conclusion without adding risk, which is why demonstration has been the standard for philosophical proof. Validity concerns the form of the inference only: a valid argument from false premises establishes nothing, so soundness, not validity alone, is what a demonstration needs.
Characteristic operations
- Fix the premises: State the starting claims explicitly, including the ones usually left unspoken.
- Check that the form preserves truth: Ensure the conclusion cannot be false while the premises are true.
- Distinguish validity from soundness: Ask separately whether the form is valid and whether the premises are actually true.
- Attack a conclusion at its source: Reject an unwanted valid conclusion by denying a premise, not the inference.
Used by traditions
Applied to questions
References
- Internet Encyclopedia of Philosophy, Deductive and Inductive Arguments
- Thomas Aquinas, Summa Theologiae I, q.2, a.3 (the five demonstrations)