 By the early 1900s, both Whitehead and Russell had completed books on the foundations of mathematics. Whitehead’s 1898 A Treatise on Universal Algebra had resulted in his election to the Royal Society. Russell’s 1903 The Principles of Mathematics had expanded on several themes initially developed by Whitehead. Russell’s book also represented a decisive break from the neo-Kantian approach to mathematics Russell had developed six years earlier in his Essay on the Foundations of Geometry. Since the research for a proposed second volume of Russell’s Principles overlapped considerably with Whitehead’s own research for a planned second volume of his Universal Algebra, the two men began collaboration on what eventually would become Principia Mathematica (1910, 1912, 1913). According to Whitehead, they initially expected the research to take about a year to complete. In the end, they worked together on the project for a decade.