The Torah says to place tefillin (leather phylacteries worn during prayer) "upon your hand" — but which hand? The Mekhilta ruled that "hand," when used without further qualification, means the left hand.
The proof comes from two biblical verses that distinguish between "hand" and "right hand" as separate terms. (Isaiah 48:13) declares, "My hand has also founded the earth, and My right hand has spanned the heavens." The verse names both — "My hand" and "My right hand" — as distinct entities. If "hand" already meant the right, there would be no need to specify "right hand" separately. Therefore "hand" alone must refer to the left.
A second proof reinforces the point from (Judges 5:26), the Song of Deborah: "Her hand reached for the tent pin, her right hand for the workmen's hammer." Again, "hand" and "right hand" appear as two different things. Yael used her left hand for one instrument and her right for another. The unqualified "hand" is the left.
This ruling shaped Jewish practice universally. Tefillin are wound on the left arm (or the right arm for a left-handed person, following additional Talmudic discussion). The left arm places the tefillin box closer to the heart, reinforcing Rabbi Yitzchak's teaching that the words of Torah should rest "upon your heart." Grammar, anatomy, and theology converge: the weaker hand binds the sign of the covenant directly against the seat of devotion.