No one knows for sure, but many scholars support the documentary theory. The documentary theory states that the author of not only Genesis, but Exodus, Leviticus, Numbers and Deuteronomy as well, used at least four independent sources. These hypothetical sources are referred to as the Yahwist source, the Elohist source, the Deuteronomist, and the Priestly source. It is common to see these sources referred to as J, E, D, and P, respectively. According to the documentary theory, the sources were written over a period of centuries, and hailed from different parts of Israel and surrounding regions.