Why do Jews believe the entire Torah, as opposed to parts or most, was given by God to Moses?

Question

It is the Rambam's 8th Principle of Faith that the entire Torah (possibly except the last several verses) was dictated by God to Moses. Why do we believe that? Why, for example, can't we say that the book of Genesis was written by Jacob, for example, and has the level of holiness and authenticity as a book of the Prophets? Perhaps the forefathers had those scriptures before Moses, and they were canonized with the rest of the Torah as a prequel? Not that they are not holy writings, but why do we have to say that Genesis was written by Moses?

Or, for example, in the last few chapters of Deuteronomy, there are discussions of Moses concluding the writing of the Torah and putting it for safekeeping among other events that would make more sense as an addendum but not as part of the Torah discussed there. Why can't we say it's included with the Torah as an addendum but not that God told that part to Moses?

Is it simply because by the time of the Gemara it was a relatively central and widespread belief, and, in the absence of any major compelling reason to say otherwise, that became recognized as a core belief?

Edit: I know that the Torah includes phrasing like "this entire book of the Torah," which the Torah says was given to the Jewish people by God through Moses. I am not asking how we know that is true. I am asking why we have to say that what we currently consider the Torah doesn't include prophetic prequels or addenda beyond what was given at Sinai.

Answer 1

Ramban in his introduction to Sefer Bereishis writes that it is true and clear that the entirety of the Torah, from the first verse to the last, was said by Hashem to Moshe to write.

אבל זה אמת וברור הוא שכל התורה מתחלת ספר בראשית עד לעיני כל ישראל נאמרה מפיו של הקב"ה לאזניו של משה, כענין שאמר להלן (ירמיה ל"ו): מפיו יקרא אלי את כל הדברים האלה ואני כותב על הספר בדיו.

However this is true and clear, that the entire Torah from the beginning of Bereishis to the [last verse of] "before the eyes of all of Israel" were said from the mouth of HaKadosh Baruch Hu into the ears of Moshe. Similar to what it says in Yirmiyahu 36: From His mouth he recites to me all these words and I write them in this sefer in ink

However, he doesn't demonstrate how he knows this to be true. Rabbeinu Bachaye to Devarim 29:6 gives an explanation, and perhaps this is where the Ramban is coming from.

שהרי קבלה בידינו שכל התורה כלה מבראשית עד לעיני כל ישראל הכל כתב משה מפי הגבורה

Behold it is a tradition in our hands that the entire Torah, from the first word Bereishis until the last words Leinei Kol Yisroel, they were all written by Moshe as directed by the Mighty One.

He says clearly that it is knowledge we have that came through mesorah, tradition. That's the source.

Answer 2

Really I think this is a very important question, one that I have struggled with.

To me, it comes down to the question of free will. If we are to accept that the whole Torah in its current form was dictated by Hashem to Moses at Sinai, then he would have been tasked with describing the events of the future, which, although we can assume that Moses would have accepted as Hashem's plan, would have been troubling for him to say the least, given the events in the Bamidbar and D'Varim.

So if we are to imagine Moses, on Sinai for forty days, grappling with visions of his own future, and the struggles described in Bamidbar / Numbers, we can believe that the Torah was revealed to Moses entirely, but not necessarily assume that Moses fully even believed all of it himself.

For example, when he and Aaron disobeyed Hashem's instructions to speak to the rock and bring forth water from it, and thereby brought about his own downfall, (which Rashi tells us was forseen by Pharoah's astrologers), we must assume that although the whole Torah was revealed to Moses, he did not fully accept the parts of it pertaining to himself, or perhaps he forgot, or in his anger/frustration, neglected what he had been instructed to do. Hence many years in the wilderness.

So the question, if I might suggest a clarification, is "in what form was the Torah given on Sinai, and to what extent did Moses record it?" This is where Christianity and Judaism split very clearly, as Christians believe that only the Ten Commandments were given at this point, which is, I believe, along the lines to your original question.

At Sinai, the Torah is not completely written down until Deuteronamy, which is three books later. So we must assume it took Moses years to express everything that had been revealed to him, and even in the end, as we saw with the water and the rock, he seemed apparently predestined to fail, and was denied access to he promised land, along with the generation that wandered in the wilderness. So there must have been some pre-and-post rationalisation on Moses part.

12 HASHEM said to Moses and to Aaron, "Because you did not believe in Me, to sanctify Me in the eyes of the Children of Israel, therefore you will not bring this congregation to the land that I have given them". 13 They are the waters of strife, where the Children of Israel contended with HASHEM, and He was sanctified through them.

So we must each ask ourselves - am I the waters of strife? Is my lack of faith going to cause unpleasant consquences like these?

This is why Jews believe that the whole Torah was revealed to Moses at Sinai, (in its entirety), in my humble opinion. Thanks for reading, I've learned a lot myself by just writing this comment.

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Answer 3

Shemot 24:12 states that God wrote the Torah. Practically speaking Torah can be divided in three parts: Bereshit - Shemot 12 (written Hebrew tradition), Shemot 12-Devarim 29:1 (Covenant at Horeb) and Devarim 29:1 - end (Covenant at Moab). We need all parts of the story to provide complete picture. Torah is about life of Moses but Genesis explains who God is, how it was created, why, e.t.c. Necessary backstory so to speak.