(Exodus 13:9) speaks of the account of the Exodus serving "as a sign upon your hand." The Mekhilta derives from this verse a specific ruling about the construction of tefillin — the leather boxes containing Torah passages that Jews bind on their arm and head during morning prayer.
The head tefillin (shel rosh) contains four separate compartments, each holding a distinct parchment inscribed with one of four Torah passages. But what about the hand tefillin (shel yad)? One might logically assume it should match: four compartments, four parchments, mirroring the head.
The Mekhilta rules otherwise. The verse says "it shall be to you as a sign" — the singular "it" indicates one unified parchment. All four Torah sections are written on a single piece of parchment and placed in a single compartment in the hand tefillin. The head tefillin has four; the hand tefillin has one.
This distinction, derived from a single pronoun in the text, became binding halakhah (Jewish religious law) observed for over two thousand years. Every pair of tefillin manufactured today follows this ruling — the box for the head divided into four compartments, the box for the arm containing one.
The symbolism runs deep. The head, seat of thought, processes the Torah's four passages as distinct ideas. The hand, instrument of action, unifies them into a single act. Thought differentiates. Action integrates. The body itself becomes a commentary on how to receive the Torah.