[From The Black Swan by Nassim Nicholas Taleb, Chapter 5]
Two statements: (1) there is no evidence of the possibility of large events; and (2) there is evidence of no possible large events.
Even though it is in fact vast, the logical distance between the two assertions will seem very narrow in your mind, so that one can be easily substituted for the other. Ten days from now, if you manage to remember the first statement at all, you will be likely to retain the second, inaccurate version–that there is proof of no Black Swans. I call this confusion the round-trip fallacy, since these statements are not interchangeable. [Taleb, pg. 52]
Still no ballerinas. 🙂