Principia Mathematica does not prove that 1 plus 1 equals 2 until well into the first volume

Alfred North Whitehead and Bertrand Russell’s “Principia Mathematica” (1910 to 1913) tried to derive the whole of pure mathematics from a small set of logical axioms, with every step written out as a formal proof that left nothing to intuition. The book is famous for its extreme rigor: simple facts were built up from the ground with painstaking care, and the proof that one plus one equals two does not arrive until well into the first volume.