Before proceeding any further, some discussion of the origins and formation of the Torah is necessary. For my Fundamentalist friends, this will not be a pleasant topic; because the evidence suggests that the Torah was formed over many hundreds of years (in other words, Moses didn't write all of it), and it did not assume its current form until sometime after the fall of the kingdoms of Israel and Judah. Nevertheless, for the benefit of my more traditionally oriented friends, I will not resort to appealing to the authority of objective Biblical scholarship on the subject (although it is substantial and approaches something close to a consensus on this topic); because most of them would dismiss such testimony out of hand. Instead, we will look at some of the internal evidence to demonstrate the validity of our thesis on this point.
First, it is clear that Moses was not an eyewitness to the events described in Genesis. The book purports to recount the history of mankind (and more particularly, the Israelites)from his creation to the time when Jacob's family moved to Egypt. The context makes clear that all of this happened prior to the birth of Moses (Exodus 2:1-10). Hence, we are left with only a few possibilities relative to the material in Genesis: 1)The account was based on oral traditions handed down from generation to generation, 2)The author consulted other documents written by other people or 3)God directly revealed this history to Moses (or whoever wrote the account). For believers, based on everything we have been able to learn about the Bible, I would say that a combination of the three is the most likely scenario.
Why not simply say that God revealed all of it to Moses? Look at the internal evidence of the book itself. There are clearly two different creation stories within the book (Compare the first chapter of Genesis to the second chapter). We read at the beginning of the fifth chapter: "This is the book of the generations of Adam..." (Genesis 5:1). Once again, clearly indicating that another "book" or document has been incorporated into the one which we have received. There is also the fact that the story of a flood is incorporated into the creation mythologies of most ancient cultures/civilizations. We also see that the story regarding King Abimelech has been applied to both Abraham (Genesis 20) and Isaac (Genesis 26), suggesting that someone has attempted to combine two competing traditions regarding these patriarchs.
In addition to these considerations, it is also highly unlikely that a humble man would have characterized himself as being more meek than anyone else on the planet (Numbers 12:3). Likewise, it is apparent that Moses could not have written the account of his own death that appears at the conclusion of the Pentateuch (Deuteronomy 34:5-12). Finally, it is apparent from subsequent references to the material attributed to Moses that it could not have approached the volume of material that we have today (notice that the entire book could be read within the context of a single public ceremony - Exodus 24:7, Joshua 8:34-35, II Kings 22:8-10, II Kings 23:2 and Nehemiah 8:3-8). So it is clear that the original material must have been augmented or embellished by others to appear as it does today (we would be hard pressed to read the entire Pentateuch in a single public ceremony today).
Hence, from the internal evidence alone, we would have to conclude that the Mosaic Law was not the work of a single man. Moreover, this conclusion also suggests that God may not have been the source for all of the laws, statutes, ordinances and judgments that appear as part of that Law!