Modus ponens vs modus tollens: Which form of argument states that if P implies Q and P is true, then Q must be true?

• A. Modus tollens
• B. Modus ponens
• C. Hypothetical syllogism
• D. Affirming the consequent

Answer: (B)

This question tests a basic rule of inference: when a conditional statement P implies Q is given, and the first part (P) is actually true, you can conclude the second part (Q). That rule is modus ponens. It’s a direct, valid move from a true antecedent to its consequent. For example, if the road is icy, the streets are slippery. The road is icy. Therefore, the streets are slippery. The pattern is P implies Q, and P is true, so Q follows.

Modus tollens, by contrast, would use a different pattern: if P implies Q and Q is not true, then P cannot be true. Hypothetical syllogism chains two conditionals to derive a new conditional, like if P implies Q and Q implies R, then P implies R. Affirming the consequent is a fallacy: if P implies Q and Q is true, you cannot conclude P.