1+1 = 2, right?

100 years ago this month, the first volume of "Principia Mathematica" was published. In this monumental undertaking Alfred North Whitehead and Bertrand Russel attempted to derive all mathematics from a simple and consistent set of axioms. It contained such notable thoughts as:

Followed by a note to the effect that "The above proposition is occasionally useful."

The authors planned a fourth volume, but confessed to being intellectually exhausted.