 Sometime during 1924, the Aufbau project changed course drastically, and arrived at essentially its later published form (Carus 2016). The phenomenological approach to the basis was dropped; a fixed “primary world” was no longer to be distinguished by phenomenological intimation from the various “secondary worlds” built upon it. Russell’s “construction principle”, as Carnap called it, came to dominate the project, and would become the book’s epigraph: 