The foundation is the commandment itself: Devarim 6:8 instructs that the words be bound as a sign on the hand and worn as a symbol between the eyes — a physical, daily act of binding that structures the body around sacred commitment.
Ein Aya, Berakhot 3:50 builds directly on this, explaining that tefillin represent strength (עז) for Israel, and that their fixed, habitual wearing — and even the careful manner of removing and storing them — trains the mind and feeling rooted in Torah to become as natural and steady as nature itself, grounded in a permanent base.
Ein Aya, Shabbat 6:20 deepens this further, showing that the very shape of the dalet of the tefillin — a halachah given to Moshe at Sinai — encodes a disposition of humility-rooted wisdom that extends strength outward to support the broken-hearted, the downtrodden, and the afflicted of the earth, in contrast to a proud wisdom that forgets those who suffer.