In my last post, I discussed how to evaluate arguments. At the end of that piece I provided some examples of formal and informal arguments. Below is the answer key for those examples.


Identify which of the following is a formal and which is an informal argument. Can you tell which of the formal arguments are valid and which are invalid?

1. Mary is sulking, and she skipped dinner. I bet she’s depressed again. [Informal]

2. Frogs are amphibians, and amphibians are vertebrates, so frogs are vertebrates. [Formal, valid]

3. All Marxists are socialists, so all socialists are Marxists. [Formal, invalid]

4. The trashcan is turned over, the garbage is everywhere, and I see paw prints. The dog must’ve gotten into the trash. [Informal]

5. If it rains, Ronny takes the bus, and I know he took the bus, so it must be raining. [Formal, invalid]

6. Rex is a dog, and no dogs can solve logic problems, so Rex can’t solve logic problems. [Formal, valid]

7. If it rains, Ronny takes the bus, and it’s not raining, so he must not be taking the bus. [Formal, invalid]

8. ‘Neo’ is the hacker alias of Thomas Anderson, and ‘Neo’ is an anagram of ‘One.’ An anagram is a word that’s produced by rearranging the letters of a different word. [Not an argument!]

9. George can’t be that serious about Sally. If he were, he’d quit messing around with Angela, and I saw the two of them heading into the Motel 6 last night. [Formal, valid]

10. Ethel is a monstrously big woman, and she does have a beard, so she really ought to join the circus. [Informal]

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