According to one opinion, the Torah was never committed to writing until the end of the fortieth year and only then was it handed to the Nation in full and finished form. A second opinion posits that the Torah was given to Moshe piecemeal over the forty years in the desert; it was made up of many different units, and in the fortieth year Moshe joined them all together, thus creating the "Torah."
From the description that appears in the Torah itself and in the Books of the Prophets, there is no way of knowing how, when, and by whom the Five Books of the Torah were committed to writing and transmitted to the Jewish People. However, in the later books the picture changes somewhat, and we find explicit mention of the existence of a "Book of the Torah" that is more extensive than just the book of Devarim. This Torah is clearly identified with "God's Torah" in Nehemia, when the Jewish people commits, at the ceremony of the covenant, "to follow God's Torah, which was given by the hand of Moshe, God's servant."
Additionally, despite the fact that the Books of the Prophets and Writings (up until Ezra and Nehemia) make no explicit mention of the existence of an extensive written Book of the Torah that goes beyond sections of Devarim, there are nevertheless clear connections throughout the Prophets and Writings to the Books of the Torah. This is clearly apparent in two phenomena: first, those instances in which these Books refer to verses in the Torah, whether openly or through allusion; and second, in the many chapters whose literary structure is built on stories from the Torah and which serve as literary parallels to them. Both phenomena are extensive in scope.
By the period of the Sages it was taken for granted in innumerable places that Moshe wrote the Torah as dictated by God. At the same time, opinions are divided as to when, and in what manner, the Torah was written and given to the Jewish people. According to one opinion, the Torah was never committed to writing until the end of the fortieth year and only then was it handed to the Nation in full and finished form. A second opinion posits that the Torah was given to Moshe piecemeal over the forty years in the desert; it was made up of many different units, and in the fortieth year Moshe joined them all together, thus creating the "Torah."
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Edited by the HaTanakh Site team