2007-06-06

Masked Cards

Here's a logic puzzle. I found it in Martin's book There Are Two Errors in the the Title of This Book, and he cites a 1968 experiment by Wason and Johnson-Laird.

You have four masked cards as shown in the image below. Which of the cards must you unmask in order to answer this question: Is it true that, in all cases, when there is a circle on the left there is a circle on the right?


Of course you can unmask all the cards and find out that way, but you don't need to unmask them all to be sure. Which cards must you unmask?

8 comments:

richard said...

I'm going with ... a and d.

No circle on the left in b, so no need to check the right side. There's already a circle on the right in c, so it doesn't matter whether there's a circle on the left in that one.

I just know I'm wrong :-)

fiona-h said...

not wrong! very good. In the book I found this in, the author said that when this puzzle was given to a group of 128 university students (in the original study), only 5 got it correct.

Mr. Kite said...

Was that 128 students at Joe's University for the mentally Ungifted?

fiona-h said...

the book didn't say, but maybe this experiment was tried at Pub Night.

Darren said...

I wants to peekin at a and d too.

lindsay said...

In this study, as I recall, everybody gets (a) but nobody gets (d). They conclude that we're sort of "wired" for Modus Ponens, but less "wired" for Modus Tollens. So we check the case "The antecedent is true." But we don't check the case "The consequent is false."

fiona-h said...

and there is a tendancy to check "C" alhtough not among this group, clearly!

(nice to see you on here)

Anonymous said...

Weird : I got "d" right away, but didn't get "a" :-(.

Mittal