Apologies to anyone whose already seen this.

The title represents four cards, each of which is known with certainty to have a letter on one side and a numerical digit on the other.

Which card(s) must one turn over to confirm that the set is compatible with the statement "All cards with an A on one side have a 7 on the other"?

Please hide answers and reasons, and mention if you've seen it before.

Easy peazy lemon undifficult

Seems straightforward - only two cards can invalidate "All cards with an A on one side have a 7 on the other" and that's the A card and the 8 card.

20 hours ago, TheVat said:

Easy peazy lemon undifficult

Interesting reply. I first encountered the problem in this video/podcast by VSauce and Hannah Fry:

If you can, I'd be interested in your critique of the relevant section (first 15 minutes or so)

For me, the interesting part was not so much as getting the right answer (I did), but knowing instinctively that I had to replay the question 3 or 4 times before I attempted it. The modus ponens part was a no-brainer, but the modus tollens took a moment or two.

Also, the transfer of the problem from an abstraction to a social rules context harked back to our discussion on the 'Johnny B Good' thread.

Will watch tomorrow - bit of a crunch this evening. Loopk forward to it - I sensed maybe I was being too glib about the puzzle's ease, so this will be interesting. Also (rusty memory of philosophy class)...ponens is affirming the if part, right? And tollens is denying the then part?) NM, I'll watch. 😀

(Full disclosure - VSauce seems like a nice guy, but his videos always get on my nerves because he is keeping the tone folksy and entertaining - it's like interacting with someone who thinks your mind is going to wander at any moment so they are obliged to tickle you.)

Watched a few minutes...

Then bailed. But got the gist I think. Most people do better with a concrete scenario with drunks or boys playing outdoors and less well with abstract. And yes, the A flip is ponens, the 8 flip is tollens, as I thought. But... only 4 percent get this?? Wow. I'm sure our SFN members would not be representative then.