Rabbi Yehudah offered a distinctive argument for the placement of the head tefillin (leather phylacteries worn during prayer), drawing an unexpected connection between the laws of tefillin and the laws of tzara'at — the skin afflictions described in Leviticus that render a person ritually impure.
His reasoning began with the established parallel between the hand and head tefillin. The Torah prescribes tefillin for both locations, so their laws should illuminate each other. The hand — specifically the skin of the upper arm — is susceptible to tumah (ritual impurity) from one type of hair abnormality: a white hair growing in a discolored skin patch.
If the hand tefillin goes on a place susceptible to impurity from one kind of hair, then the head tefillin should likewise go on a place susceptible to impurity from one kind of hair. The top of the head, where scalp afflictions are examined for a single sign — a yellow hair — fits this criterion.
The area between the eyes, by contrast, is susceptible to impurity from two kinds of hair signs, making it a poor match for the parallel. Rabbi Yehudah's argument therefore eliminates the literal "between the eyes" and confirms the top of the head as the correct placement.
This is a characteristically Tannaitic move — connecting two apparently unrelated areas of law through structural analogy. Tefillin and skin disease seem to have nothing in common, yet Rabbi Yehudah found a hidden thread linking them. The body, in rabbinic thought, is a unified legal landscape where the rules governing one limb shed light on the obligations of another.