Why is the Torah divided into five books?

Question

When the Torah refers to itself, it refers to itself in the singular. There are numerous verses, notably in Devarim. One notable example Devarim 31:16 (Sefaria English translation:)

"לָקֹחַ אֵת סֵפֶר הַתּוֹרָה הַזֶּה וְשַׂמְתֶּם אֹתוֹ מִצַּד אֲרוֹן בְּרִית־ה' אֱלֹקיכֶם וְהָיָה־שָׁם בְּךָ לְעֵד׃"

Take this book of Teaching and place it beside the Ark of the Covenant of the LORD your God, and let it remain there as a witness against you.

Yet, we see that there are five sefarim (books) and on parchment, we denote the separate books by leaving several lines of spacing between the end of one book and the start of the next.

I notice that Rash"i as well as the Gemarah refer to some of the separate books by names such as "Torat Kohanim" (Vayikra) and "Mishneh Torah" (for Devarim).

When did the concept of separating the Torah into Five books begin? And, why was there a requirement to write a Torah into five books? Is that Misinai (from Sinai)? It doesn't appear to be this way from the Torah's verbage. Even it is Misnai, why separate it into five books?

Answer 1

The first reference to the five-sefarim division of the Torah goes back to Philo (ca. 25 B.C.E.- 50 C.E.) in the opening of his De Abrahamo, in which he states that the Torah has five books:

τῶν ἱερῶν νόμων ἐν πέντε βίβλοις ἀναγραφέντων ἡ πρώτη καλεῖται καὶ ἐπιγράφεται Γένεσις ἀπὸ τῆς τοῦ κόσμου γενέσεως

The first of the five books in which the holy laws are written bears the name and inscription of Genesis, from the genesis or creation of the world, an account of which it contains at its beginning (Translation by F. H. Colson).

He also declares the same in the De Aeternitate Mundi.

Decades later, Josephus in Against Apion, 1:8 states:

οὐ μυριάδες βιβλίων εἰσὶ παρ᾽ ἡμῖν ἀσυμφώνων καὶ μαχομένων, δύο δὲ μόνα πρὸς τοῖς εἴκοσι βιβλία τοῦ παντὸς ἔχοντα χρόνου τὴν ἀναγραφήν, τὰ δικαίως πεπιστευμένα. καὶ τούτων πέντε μέν ἐστι Μωυσέως, ἃ τούς τε νόμους περιέχει καὶ τὴν ἀπ᾽ ἀνθρωπογονίας παράδοσιν μέχρι τῆς αὐτοῦ τελευτῆς

Our books, those which are justly accredited, are but two and twenty, and contain the record of all time. Of these, five are the books of Moses, comprising the laws and the traditional history from the birth of man down to the death of the lawgiver. (Translation by Henry St. John Thackeray)

The same expression is later found in the Yerushalmi (see Sotah 26a), Otios de Rabi Akiva (Nusach Alef), the Medrash Shocher Tov (and etc).

Despite these sources do not explain the reason behind that, it resembles to have something related with its content. This is what Jeffrey Tigay presents,

“The Torah in its final form is divided into five separate books because ancient scrolls could not contain a work of that length. Nevertheless, the books were not divided arbitrarily but at natural transition points.” (“Exodus,” in The Jewish Study Bible, 2nd ed. (New York: Oxford University Press, 2014), 95)

Answer 2

Tikuney Zohar (25) seems to equate the number of books in the Torah with the five ‘wings’ of an animal’s lungs. Saying too, that a Torah must be exactly perfect not more, less or exchanged. So to the lungs, if they are additional or lacking the animal is invalid.

And one who transgresses one has certainly the other.

כְּגַוְונָא דָא אִינוּן חָמֵשׁ כַּנְפֵי רֵיאָה, כְּגַוְונָא דַחֲמִשָּׁה חוּמְשֵׁי תוֹרָה, וּוַרְדָּא כְּגַוְונָא דְזֶה סֵפֶר דְּאִיהוּ סֵפֶר יְשָׁרִים, אִי אוּנֵי חַסִּיר אוֹ יַתִּיר אוֹ חֲלִיף, אִיהוּ אִמְרָא פָּסוּל, וּמָאן דְּעָבַר עֲלַיְיהוּ כְּאִלּוּ עָבַר עַל אוֹרַיְיתָא וַחֲמִשָּׁה חוּמְשֵׁי דִילָהּ, דַּחֲמִשָּׁה כַנְפֵי רֵיאָה וּוַרְדָּא אִינוּן שִׁית, דָּא ו', כָּל מָאן דְּעָבַר (על דא, כאלו עבר) עַל דָּא.

Answer 3

According to בית אלהים, שער היסודות פרק לב, there are several possibilities.

Five book correlate to Adam, Noach, Avraham, Yitzchak, Yaakov.

Or: Amraam, Yocheved, Aharon, Miriam, Moshe.

Alternatively corresponding to five places: Mitzrayim, Marah, Har Sinai, Ohel Moed, Aravos Moav.