Book of Deuteronomy have given various answers to the problem. The Deuteronomistic history theory is currently the most popular (Deuteronomy was originally just the law code and covenant, written to cement the religious reforms of Josiah, and later expanded to stand as the introduction to the full history); but there is an older theory which sees Deuteronomy as belonging to Numbers, and Joshua as a sort of supplement to it. This idea still has supporters, but the mainstream understanding is that Deuteronomy, after becoming the introduction to the history, was later detached from it and included with Genesis-Exodus-Leviticus-Numbers because it already had Moses
Development of the Old Testament canon Church, first found as last chapter of the eighth book of the Apostolic Constitutions. Canon n. 85 of the "Ecclesiastical Canons of the Same Holy Apostles" is a list of canonical books, includes 46 books of Old Testament canon which essentially corresponds to that of the Septuagint. The Old Testament part of the Canon n. 85 stated as follows: Karl Josef von Hefele argues that "This is probably the least ancient canon in the whole collection"; even he and William Beveridge believe that the writings of the Apostolic Canons dating from end of the second or early of the third