Excerpted and adapted from a shiur by HaRav Chaim Weg
July 29, 2020
And you shall tie them for a sign on your arm, and they shall be as totafos between your eyes.
The Rambam (Hilchos Tefillin 4:1) writes that the tefillin must be centered on the head (“tzarich l’chavein b’emtza”). What is the source of this halacha? Perhaps we can suggest the following explanation.
The Gemara (Menachos 34b) states that the order of the parshiyos in the shel rosh is Kadesh li and V’haya ki yeviacha on the right and Shema and V’haya im shamoa on the left, and “hakorei korei k’sidran,” one who reads, reads them in order. According to Rashi, the right and left and korei refer to the right and left of one facing the wearer of the tefillin. Thus, he would read the parshiyos from right to left in the order they are written in the Torah: Kadesh, V’haya, Shema, and V’haya im shamoa. Rabbeinu Tam interprets the description differently and arrives at the sequence—from the right to the left of one facing the wearer—of Kadesh, V’haya, V’haya im shamoa, and Shema.
Why did the breisa formulate the order based on a division between the right side and the left side? Couldn’t it have simply written the order of the parshiyos without making this division? The Piskei Siddur in the back of the Shulchan Aruch HaRav explains that this teaches that the right two parshiyos must be on the right side of the head and the left two must be on the left side of the head. This, then, may be the source of the halacha that the tefillin must be centered.