The Mekhilta DeRabbi Yishmael derives a striking equivalence from the verse "and as a remembrance between your eyes, so that the Torah of the L-rd be in your mouth" (Exodus 13:9). This verse links two things: wearing tefillin (leather phylacteries worn during prayer) between your eyes and having the Torah in your mouth. From this linkage, the rabbis drew a legal ruling that shaped the daily practice of Jewish worship.
The ruling states: if a person wears tefillin, it is as if he is reading the Torah. The physical act of binding the leather boxes to one's head and arm, with their enclosed parchments inscribed with Torah passages, is itself a form of Torah engagement. The tefillin are not merely a reminder to study. They are, in a legal sense, equivalent to study itself.
The Mekhilta then extends this principle in the reverse direction: one who is actively reading the Torah is exempt from the obligation of tefillin. If reading Torah and wearing tefillin are legally equivalent, then performing one satisfies the obligation of the other. A person engaged in Torah study has already fulfilled what tefillin represent.
This ruling reveals something profound about how the rabbis understood the purpose of tefillin. They are not ornamental. They are not merely symbolic. They are a physical form of Torah study, a way of having the Torah "in your mouth" through binding it to your body. The verse in Exodus creates an equation: tefillin equals Torah in the mouth. The rabbis took this equation literally and built law from it.
In practice, the ruling about exemption during Torah study is largely theoretical, since the standard halakhah (Jewish religious law) requires wearing tefillin during the morning service regardless. But the Mekhilta's teaching preserves the original logic: that wearing tefillin and engaging with Torah are not two separate commandments but two expressions of the same sacred act.