I am of the view that Moses wrote the entire Torah, with a few exceptions and caveats to be noted shortly. However, this does not mean that Moses wrote these five books of the Pentateuch in the same way and style. For example - see the appendix on the most plausible way that Moses probably wrote Genesis.

Unlike Genesis, Exodus, Leviticus and Numbers were written by Moses for the reasons to be stated shortly. That while Genesis was compiled and edited, Exodus, Leviticus and Numbers appear to be original works of Moses.

Deuteronomy is clearly different because it consists of four speeches by Moses which means we have two possible ways it was recorded; either

• Moses recorded and wrote the book (except the last chapter) just before he died about 1400 BC, or,
• [more probably??] Someone like Joshua recorded Moses' speeches, adding an introduction and final chapter about Moses' death.

In either case, Moses is the author, just as Jeremiah was the author of his book and used Baruch as his secretary (Jer 32:12, 36:4, 14, 32, 45:1, etc.)

I believe Moses composed the book of Deuteronomy for the following reasons:

1. Moses is the traditional author, without dissent, until the 19th century. Thus, tradition is overwhelmingly in favor of Moses as author
2. The NT authors appear to confirm this unanimously, Matt 19:7, 8 (quoting Deut 24:1), Mark 12:19 (quoting Deut 25:5), John 1:17, 7:19 (quoting Deut 4:44), John 1:45, Acts 3:22 (quoting Deut 18:15), Heb 10:28 (quoting Deut 17:2-7)
3. The form of Hebrew and its expressions is very old Hebrew, discernibly different from Hebrew of later centuries [I am no expert in this area.]
4. A number of "irregularities" testify to an ancient origin as well such as the differences in the 10 commandments between Ex 20 and Deut 5 - if this is the product of a later author/editor, these records would be more uniform.

None of this is to say that the text of the Torah was not altered in (hopefully) minor ways in later centuries - the extent of which is almost impossible to know but it is possible to see some later changes such as place names for some cities and locations.

APPENDIX - Origin of Genesis

Almost all commentators now structure the literary form of the book of Genesis around the “Toledoths” – a Hebrew word that is variously translated as, “the generations of”, “the history of”, “the account of”, “the record of”, etc. The debate in Genesis concerns the function of these Toledoths – do they form a heading (for what follows) or a colophon (footnote and “signature” of what has preceded)? Wiseman [Wiseman, P. J. (1936). New Discoveries in Babylonia about Genesis. London: Marsh, Morgan and Scott.] suggested, after studying Akkadian documents, that these Toledoths were colophons containing the identity of the author, and created an elaborate Tablet theory about Genesis. What are the facts? The 11 Toledoths in Genesis are:

1. Gen 2:4 toledoth of Heavens and Earth
2. Gen 5:1 toledoth of Adam
3. Gen 6:9 toledoth of Noah
4. Gen 10:1 toledoth of Shem Ham and Japheth
5. Gen 11:10 toledoth of Shem
6. Gen 11:27 toledoth of Terah
7. Gen 25:12 toledoth of Ishmael
8. Gen 25:19 toledoth of Isaac
9. Gen 36:1 toledoth of Esau
10. Gen 36:9 toledoth of Esau in Hill Country
11. Gen 37:2 toledoth of Jacob

If the Toledoths are intended as a Colophon (footnote containing the author’s “signature”) to each section, then:

• The last third of Genesis has no author
• Some sections were written by people who did not witness the events or were antagonistic to the events and people, eg, most of the last 5 except Gen 36:9.
• The first section was written by the heavens and the earth (!?!)

By contrast, if each Toledoth is a section heading:

• Each section is about what the heading states without exception
• Gen 1:1 – 2:3 is clearly written separately and in a very different style, verging on poetic.

This leads to a simple idea about the possible origin of the book of Genesis. It is probable (in agreement with Wiseman) that each Toledoth was written by a different (unstated) person (NOT the person in the Toledoth), and later, Moses collected and collated them, edited and arranged them, with an added introduction about creation, to create the book of Genesis, essentially as we now have it.