People who claim that they can knit but cannot purl are truly incredible. It's like:

--playing tennis and not being capable of backhand strokes.
--writing from the point of view of a man and not being capable of writing from the point of view of a woman.
--being able to inhale but not being able to exhale.
--playing Scrabble, forming words, and not being capable of reversing those words.
--being able to perform onstage but not being able to work backstage and in the wings.

I've heard of writer's block, but purling block is quite unbelievable.

If you can multiply but not divide, all you have to do is use more hours practicing division.

If you can knit but "cannot" purl, it is possible that, the truth is:

--you are lazy to learn.
--you have poor motor coordination.
--you are exercising your defense mechanisms against the act of knitting.