Freud had been in private practice for nine years when he wrote to Fliess on May 25, 1895 of the inhuman amount he had to do and of his long hours working with neuroses. He was engaged at that time—and throughout his entire working life—in an attempt to theorize his day to day practical activity. In this chapter we will trace the path he forged over many decades, the focus being his effort to conceptualize the split psyche, his fundamental axiom.