Rabbi Eliezer agreed that the tefillin (leather phylacteries worn during prayer) belong on the upper arm rather than the palm, but he arrived at the conclusion through entirely different reasoning. His argument hinged on the word "sign" in (Exodus 13:9): "It shall be to you as a sign upon your hand."
The critical phrase is "to you as a sign." Rabbi Eliezer read this as "to you as a sign, and not to others as a sign." The tefillin on the arm are meant to be a private sign between the wearer and God — visible to the one who wears them but not displayed publicly to everyone else.
This distinction between the palm and the upper arm is practical. The palm of the hand is one of the most visible parts of the body. It faces outward during conversation, gesture, and daily activity. The upper arm, by contrast, is typically covered by clothing. When the tefillin are placed on the bicep, they remain concealed — a sign known to the wearer and to God, not a display for others.
Rabbi Eliezer's interpretation adds a dimension of intimacy to the commandment. The head tefillin are visible to all — they sit prominently on the forehead. But the arm tefillin are hidden. One sign is public declaration. The other is private devotion. Together they represent a relationship with God that operates on both levels: the faith you show the world and the faith you keep between yourself and your Creator, hidden beneath your sleeve.