I submitted a poll,
http://news.ycombinator.com/item?id=2124808,
then realised the short-coming that people would see the current voting before casting a vote.
Here's a quick experimental hack as an alternative.
In the light of a recent article (http://news.ycombinator.com/item?id=2121428) by Tim Gowers about the difficulties some people have with mathematical logic, I wondered what people here would answer to this question.
Suppose I have four cards. Each card has a letter on one side, and a digit on the other. I arrange them so that you can see "D", "3", "K" and "4". I then claim that every "D" has a "4" on the other side.
Which cards do you need to turn over to see if I'm right?
Click on the appropriate link.
Just D - http://www.solipsys.co.uk/new/LogicalImplication.html?D
Just 3 - http://www.solipsys.co.uk/new/LogicalImplication.html?3
Just K - http://www.solipsys.co.uk/new/LogicalImplication.html?K
Just 4 - http://www.solipsys.co.uk/new/LogicalImplication.html?4
D and 3 - http://www.solipsys.co.uk/new/LogicalImplication.html?D3
D and K - http://www.solipsys.co.uk/new/LogicalImplication.html?DK
D and 4 - http://www.solipsys.co.uk/new/LogicalImplication.html?D4
3 and K - http://www.solipsys.co.uk/new/LogicalImplication.html?3K
3 and 4 - http://www.solipsys.co.uk/new/LogicalImplication.html?34
K and 4 - http://www.solipsys.co.uk/new/LogicalImplication.html?K4
D, 3 and K - http://www.solipsys.co.uk/new/LogicalImplication.html?D3K
D, 3 and 4 - http://www.solipsys.co.uk/new/LogicalImplication.html?D34
D, K and 4 - http://www.solipsys.co.uk/new/LogicalImplication.html?DK4
3, K and 4 - http://www.solipsys.co.uk/new/LogicalImplication.html?3K4
All of them - http://www.solipsys.co.uk/new/LogicalImplication.html?D3K4