 With the help of Whitehead, Russell extended Peano’s symbolic logic in order to be able to deal with all types of relations and, consequently, with all the patterns of relatedness that mathematicians study. In his Principles of Mathematics, Russell gave an account of the resulting new symbolic logic of classes and relations—called ‘mathematical logic’—as well as an outline of how to reconstruct all existing mathematics by means of this logic. After that, instead of only being a driving force behind the scenes, Whitehead became the public co-author of Russell of the actual and rigorous reconstruction of mathematics from logic. Russell often presented this reconstruction—giving rise to the publication of the three Principia Mathematica volumes—as the reduction of mathematics to logic, both qua definitions and qua proofs. And since the 1920s, following Rudolf Carnap, Whitehead and Russell’s project as well as similar reduction-to-logic projects, including the earlier project of Gottlob Frege, are classified under the header of ‘logicism’.