A discussion I had in the reddit comments on that Slate post made me invent this fake argument:
A: People who drink water inevitably end up dead. Therefore drinking water causes death.
B: No, that is correlation, not causation.
C: No, it is not correlation. To calculate correlation you divide the covariance of the two variables by the variance of each of the variables. In this case there is no variance in either variable, so you’re dividing by zero, so correlation is not even defined.
I think it’s an improvement to go from saying “there is obviously something wrong with A’s argument” to actually being able to point out the divide-by-zero in the equation.
Oh god. Everyone stop talking.