On a New York sidewalk today I passed a stand selling baby outfits with this message printed in big letters across the chest,

“If I don’t sleep, nobody sleeps!”

We have here a fine case study in logic. The negatives on both sides are a tipoff that there’s a simpler equivalent formulation, the contrapositive,

“If somebody sleeps, I sleep!”

So why does this one feel so wrong? It’s the distinction between implication and causation. Yes, if somebody sleeps, that logically implies that baby must sleep. But it doesn’t cause baby to sleep, and indeed it’s baby who must drift off first before anyone else will get a chance. “If I don’t sleep…” says it right because it aligns the logic and the causation. Maybe it adds to the fun that we dimly sense this twist.

[29 September 2014]