Logic / Argument Forms
Least You Need to Know: Argument Forms
A valid argument form guarantees the conclusion whenever the premises are true. Learn the classic valid forms and the common traps.
Least you need to know
- Modus ponens and modus tollens are valid.
- Hypothetical syllogism chains implications.
- Affirming the consequent is invalid.
- Denying the antecedent is invalid.
- Validity depends on the form, not on whether the premises are actually true.
Key notation
- p → q — if p then q
- ¬q — not q
- ∴ — therefore
- p — premise or conclusion statement
Worked example
- Premises:
p → q,q → r, andp. - First use modus ponens to get
q. - Then use modus ponens again to get
r. - So the chained argument is valid.
Common mistakes
- Students often treat
qas enough to concludepfromp → q. - Students sometimes label an argument invalid because the topic sounds unfamiliar.
- Students often mix up modus tollens with denying the antecedent.
How to recognize it
- The problem gives premises and asks whether the conclusion follows.
- You see an implication plus either its premise or the negation of its conclusion.
- The answer choices describe named argument forms.
บทเรียนถัดไปที่แนะนำ
เรียนต่อในหัวข้อนี้ด้วย Least You Need to Know: Equivalence Laws
Least You Need to Know: Equivalence Lawsบทเรียนที่เกี่ยวข้อง
เรียนต่อด้วยบทเรียนใกล้เคียงในหัวข้อเดียวกัน