In this paper I explain the most important principle of research I followed in achieving my study on the life of Muhammad. The literary genre of hadith which did not come into being before the end of the seventh century AC must not be considered as a reliable source for the study of Muhammad's biography, since it reflects the theological and social developments of that subsequent epoch. Therefore research on the historical Muhammad has to be based on the koran.
The text of the koran embodies the evidence of a prophetic experience which lasted for more than two decades. The different meanings of rasūl and nabī are the starting-point of the exposition of the method underlying my biography of Muhammad. Some of the major consequences of this difference are analyzed, and I try to show its momentousness for understanding the historical prophet.