tremendous question of the Chasam Sofer on the mitzvah of kesivas sefer Torah

The Chasam Sofer (Shu”T Y.D. 254) writes that he asked many gedolim the following question and he never got an answer that satisfied him: the gemara (Gitin 60) has a machlokes whether Torah was given in one chunk or was given parsha by parsha.  Nafka minah: is whether one is permitted to write a single parsha of Torah or must Torah be written as one whole, in its entirety, all or nothing?

If, as we pasken, the Torah must be written as a whole and not in units of parshiyos, how can we possibly fulfill the mitzvah of tefillin or the mitzvah of mezuzah?  Don’t these mitzvos require writing specific parshiyos?

The obvious answer is that by definition the mitzvah of tefillin or mezuzah includes a matir to write individual parshiyos.

The Rambam (Hil Sefer Torah 7:1) formulates the mitzvah of writing a sefer Torah as follows:

 

מצות עשה על כל איש ואיש מישראל, לכתוב ספר תורה לעצמו: שנאמר "ועתה, כתבו לכם את השירה הזאת" (דברים לא,יט), כלומר כתבו את התורה שיש בה שירה זו--לפי שאין כותבין את התורה, פרשייות פרשייות.

The word “shirah” used in the command “kisvu lachem es ha’shirah ha’zos” technically refers only to parshas ha’azinu.  However, explains the Rambam, since one is not permitted to write individual parshiyos, m’meila the command requires of us to write the entire Torah.

Why, asks the Chasam Sofer, is the mitzvah of “kisvu lachem es ha’shirah ha’zos” different than the mitzvah of tefillin or mezuzah?  Just like in the latter cases we understand that the mitzvah by definition allows the writing of individual parshiyos, so too, perhaps “kisvu lachem es ha’shirah ha’zos” allows by definition the writing of ha’azinu as an isolated parsha? 

 

(The Chasam Sofer gives an answer, but I don't want to spoil your enjoyment of puzzling over the question.)