Richard Bell took Nöldeke's chronology as a starting point for his research, however, Bell did not believe that Nöldeke's criteria of style were important. He saw a progressive change in Muhammad's mission from a man who preached monotheism into an independent leader of a paramount religion. For Bell this transformation in Muhammad's mission was more decisive compared with Nöldeke's criteria of style. Bell argued that passages which mentioned Islam and Muslim or implied that Muhammad's followers were a distinct community were revealed later. He classified the Quran into three main periods: the early period, the Quranic period, and the book period. Richard bell worked on the chronology of verses instead of chapters. Underlying Bell's method for dating revelations is the assumption that the normal unit of revelation is the short passage and the passages have been extensively edited and rearranged.