Jewish tradition holds that the Torah was given to Moses at Sinai, written down by him, and transmitted intact, a position the Mishnah states at the opening of Pirkei Avot and Maimonides made the eighth of his thirteen principles of faith. Christian tradition inherited the same attribution. Orthodox Judaism continues to hold Mosaic authorship as doctrine, and it remains the working assumption of most traditional commentary.
From the eighteenth century onward, readers noticed features that are hard to reconcile with a single author: two creation accounts, two interleaved flood narratives with different animal counts and durations, alternating divine names, repeated stories with changed details, and an account of Moses's own death and burial. The classical answer was the documentary hypothesis, given its best-known form by Julius Wellhausen in 1878, which separated four continuous sources, J (using the divine name Yahweh), E (using Elohim), D (Deuteronomy) and P (the priestly material), and dated them across the monarchy and exile.
That four-source model is no longer the consensus it was for most of the twentieth century. Since the 1970s, work by Rolf Rendtorff, Erhard Blum, John Van Seters and others has argued that J and E in particular do not hold up as continuous documents, and supplementary models (a core text expanded by successive editors) and fragmentary models (independent blocks joined late) now compete with it, especially in European scholarship. A neo-documentarian school, strong in North America and Israel, defends a revised source division. What almost all critical scholars share is the conclusion that the Torah is composite and that the priestly material is distinguishable; where it came from and when is genuinely unsettled.