The Mekhilta DeRabbi Yishmael records a direct question from Rabbi Yitzchak: where exactly does the tefillin go?
The Torah commands "on your hand" (Exodus 13:16), which could be read literally as the palm or anywhere on the hand. Rabbi Yitzchak asks: is it really on the upper arm, or perhaps on the palm itself? If we read only this verse, the answer is ambiguous. The Hebrew word "yad" can refer to the entire arm or just the hand.
To resolve the question, Rabbi Yitzchak turns to a parallel verse in (Deuteronomy 11:18): "And you shall place these words upon your heart." This verse does not mention the hand at all. It speaks of the heart. And by comparing the two verses, the rabbis derived their answer: the tefillin must be placed on the part of the arm that is "aligned with your heart," meaning the upper arm, specifically the bicep area of the left arm, which sits directly opposite the heart.
This method of interpretation, harmonizing two verses to produce a legal ruling neither verse states on its own, is one of the hallmarks of rabbinic exegesis. Neither Exodus nor Deuteronomy says "put tefillin on your upper left arm near the heart." But by reading them together, the rabbis arrived at a practice that has remained unchanged for over two thousand years. Every morning, Jewish men wrap the tefillin strap around the left bicep, positioning the box as close to the heart as possible, exactly as Rabbi Yitzchak derived from these ancient verses.