"If one eats mushrooms after a long gap of time, one falls ill. I have been…
2024
"If one eats mushrooms after a long gap of time, one falls ill. I have been having some digestive issues since morning today, therefore there must have been mushroom in that mixed vegetable soup we had last night." Which fallacy, if any, is committed in the above argument?
Answer: B. Affirming the consequent — ConceptA conditional "If P, then Q" asserts only that P is sufficient for Q; it never asserts that P is necessary for Q. Because Q may follow from many other…
- A.
It is a valid argument
- B.
Affirming the consequent
- C.
Hasty Generalization
- D.
Begging the question
Show answer & explanation
Correct answer: B
Concept
A conditional "If P, then Q" asserts only that P is sufficient for Q; it never asserts that P is necessary for Q. Because Q may follow from many other sufficient conditions, the truth of Q leaves P undetermined. The pattern "If P then Q; Q; therefore P" is consequently invalid, and that pattern carries the name affirming the consequent. It is a formal fallacy: the defect lies in the shape of the argument, so it persists whatever P and Q stand for.
Application
Identify the two parts of the conditional premise: the antecedent P is "one eats mushrooms after a long gap of time" and the consequent Q is "one falls ill".
The first premise states the conditional itself: If P, then Q.
The second premise reports the speaker's digestive trouble since this morning, which the argument treats as an instance of falling ill — that is, it asserts Q.
The conclusion claims that last night's mixed vegetable soup contained mushroom, which within the argument stands for the speaker having eaten mushrooms after a long gap — that is, it asserts P.
Both of those readings are implicit bridges that the passage never spells out; grant them, and the argument is being taken at its strongest.
Even granted, the form is: If P then Q; Q; therefore P. The antecedent has been read back out of the consequent, and that is the invalid step.
Cross-check
Test the form against a counter-case: digestive trouble can equally follow from stale food, an infection or simple over-eating, so Q can be true while P is false. The premises can therefore all hold while the conclusion fails, which is precisely what invalidity means. Set the pattern beside the two valid conditional forms:
Argument form | Pattern | Status |
|---|---|---|
Modus ponens | If P then Q; P; therefore Q | Valid |
Modus tollens | If P then Q; not-Q; therefore not-P | Valid |
Affirming the consequent | If P then Q; Q; therefore P | Invalid |
The passage instantiates the pattern "If P then Q; Q; therefore P", and so matches neither of the two valid forms.
Why the other labels do not fit
Valid argument: validity would require that the premises cannot all be true while the conclusion is false. Here they can, so the reasoning is not valid.
Hasty generalization: that error runs from a small or unrepresentative sample up to a broad general rule. This passage runs the other way, from a general rule down to a particular claim about last night's soup.
Begging the question: that error smuggles the conclusion into the premises. Here the conclusion, that the soup contained mushroom, appears nowhere among the premises, so the reasoning is not circular.
Result
The argument commits the formal fallacy of affirming the consequent.