Abaye raised a dilemma before Rabba: What is the halakha with regard to whether it is permitted to write a scroll containing only one portion of the Torah for the purpose of enabling a child to study it? The Gemara notes: Let the dilemma be raised according to the one who says that the Torah was given from the outset scroll by scroll, meaning that Moses would teach the Jewish people one portion of the Torah, and then write it down, and then teach them the next portion of the Torah, and then write that down, and continue in this way until he committed the entire Torah to writing. And let the dilemma also be raised according to the one who says that the Torah was given as a complete book, meaning that the Torah was not written down incrementally, but rather, after teaching the Jewish people the entire Torah, Moses committed it to writing all at once.

The Gemara explains the two sides of the dilemma according to each opinion: Let the dilemma be raised according to the one who says that the Torah was given scroll by scroll. On the one hand it is possible to say that since the Torah was originally given scroll by scroll, today as well one may write the Torah in separate scrolls. Or on the other hand, perhaps one should say that since it was ultimately joined together to form a single scroll, it was joined together and can no longer be written in separate scrolls.

And let the dilemma also be raised according to the one who says that the Torah was given as a complete book. On the one hand it is possible to say that since it was given from the outset as a complete book, one may not write it today in separate scrolls. Or on the other hand, perhaps one could say that since it is not always possible to write a complete Torah, one may write it in separate scrolls. Rabba said to him: One may not write the Torah in separate scrolls. And what is the reason? Because one may not write a scroll that is only part of the Torah.

scroll -- one or two parshas/sections of the Torah

In separate scrolls - When each portion was told to Moses, he wrote it down and, after 40 years when all the portions were completed, he attached them with sinews and bound them.

~ trans. Sefaria

"‘Given in complete form’ – [This means that] it was not written until the end of the forty years, after all the sections had been spoken [by God]. And the portions that were spoken to him during the first and second years were retained by him orally until he wrote them down." ~ trans. ChatGPT