7 replies to this topic

[Brian Hunt]

My friend posted the following on facebook:

"All Cats are Animals, and all Dogs are Animals, and all Kangaroos are Animals, so therefore all Cats are Dogs, and all Dogs are Cats, but not all Dogs are Kangaroos, and Kangaroos can only be Cats if the Dogs aren't Cats. Disprove me"

I told him his logic was faulty and I don't feel like typing the reasons why because I've typed them oh so many times, and I feel they're obvious but he keeps insisting that he is correct.

Any thoughts? My mind is exhausted as I've tried to explain it for over two hours.

[haley.the.great]

My thoughts are, your friend is successful.

[Avan:)]

everything after "all kangaroos are animals" is wrong, as the therefore is not justified and is not valid?
  i think. argh...i hate tok.

[Sandwich]

"All Cats are Animals, and all Dogs are Animals, and all Kangaroos are Animals, so therefore all Cats are Dogs, and all Dogs are Cats, but not all Dogs are Kangaroos, and Kangaroos can only be Cats if the Dogs aren't Cats. Disprove me"

Because therefore all A are B is not a logical progression of the statements all A are C and all B are C.

Unless, of course, you're arguing for religion in which case, logic can go run out the window, everything can be C...

[Proletariat]

Your friend's trying to have a good chuckle. In reality, he/she isn't even discussing logic, but rather semantics. The word "animals" is used in the adjective form, not the noun. Therefore, the ability of "Cats" and "Dogs" to share a single similar quality does not make them identical entities, and the rest of the statement does not logically follow. To illustrate the fallacy, simply replace the word "animals" in that phrase with the word "red."

If your friend insists that this is deductive logic though, then he/she still has not presented a valid argument; see Sandwich's response =P

Edited by Proletariat, Jul 04, 2011 - 18:06.

[Brian Hunt]

He believes this is Deductive logic. He doesn't even know the word semantics. I've been through it multiple times, and even drew a diagram but he insists his deductive logic is valid.

[Proletariat]

Brian Hunt, on Jul 04, 2011 - 17:29, said:

He believes this is Deductive logic. He doesn't even know the word semantics. I've been through it multiple times, and even drew a diagram but he insists his deductive logic is valid.

Say to your friend "My nose is made of cheese. Disprove me." Then promptly ignore every single attempt at refutation.

[Sandwich]

Or just tell him he's a twerp*

* 4 letter word rhyming with 'bat', actually, but IBS censored it