Deductive reasoning aims to show that the conclusion must be true if the premises are true. It deals with certainty.
Key Concepts
Validity
An argument is valid if the structure is such that it is impossible for the premises to be true and the conclusion false. Validity is about the form, not the content.
Valid Structure:
If P, then Q. P. Therefore, Q. (Modus Ponens)
Soundness
An argument is sound if it is:
- Valid
- All its premises are actually true.
Sound = Valid + True Premises
Common Valid Forms
- Modus Ponens: If P then Q; P; therefore Q.
- Modus Tollens: If P then Q; Not Q; therefore Not P.
- Disjunctive Syllogism: P or Q; Not P; therefore Q.