The Chabad reading, and its date

The reading most people who grew up with an explanation of tefillin were given: the head-box is the intellect and the arm-box, opposite the heart, is the emotions, and binding them in that order enacts the rule of mind over feeling — mochin governing middot.

In Chabad thought this is precise rather than poetic. The intellect is three faculties, Chochmah–Binah–Da'at, and the emotional attributes below are seven; and the Tanya calls the three the "mothers" of the seven, because an emotion that is not the offspring of an understanding is held to be an unstable thing. Bind the head first in intention and the arm follows.

It is a genuinely powerful reading and it does land on three and seven. Note where it lands, though: on the faculties, not on the compartments — the box above still has four. And note the date. The Tanya is 1797. This is a reading roughly two centuries old, laid over an object two millennia older, and it is the reading that has largely displaced all the others in popular explanation.

⚠ A word about a coincidence, stated and then left alone: the Tanya calls the three faculties the mothers, and the Sefer Yetzirah calls three letters the mothers. The word is shared and the number is shared. Whether that is an allusion, an inheritance through the intervening Kabbalistic literature, or two independent uses of an obvious metaphor, this app does not know and will not assert. The shared word is a fact; the intention behind it is not one.