Many years ago I picked up a copy of Martin Gardner’s book *A Gardner’s Workout.* In his book, Gardner reviews a collection of writing by the late Ralph Boas, a former professor and department chair at Northwestern University.

In his review of of this collection, Gardner says:

In Princeton’s Fine Hall, Boas recalls, someone once posted a “Scale of Obviousness”:

If Wedderburn says it’s obvious, everybody in the room has seen it ten minutes ago.

If Bohnenblust says it’s obvious, it’s obvious.

If Bochner says it’s obvious, you can figure it out in half an hour.

If von Neumann says it’s obvious, you can prove it in three months if you’re a genius.

If Lefschetz says it’s obvious, it’s wrong.

What I love about this little bit of mathematical humor is that the idea of an “obvious” proposition in mathematics is so inherently subjective. In fact, in grad school (when I first read this) we had our own joke, which was that if we didn’t know how to justify some part of the proof we were working on, we should just write “clearly.” Or “the proof is left to the reader.”

On top of the humor is the historical nature of this little joke. The people mentioned are all well-known mid-century mathematicians from Princeton University faculty.