The people of Israel once came before God with a complaint that only a working people could make. Rabbi Eliezer preserved their words: "We are anxious to be occupied day and night in the Torah, but we have not the leisure. We must plow, we must trade, we must sleep."
The Holy One — blessed be He — did not dismiss them. He answered with a gift. "Perform the commandment of the phylacteries," He said, "and I will count it as if you were occupied day and night in the Torah." The black box bound to the arm and the head, the small scroll pressed against the heart and the temple — these become, in heaven's accounting, the whole night of study the laborer cannot sit through.
The Yalkut Shimoni preserves this teaching with a related one: tefillin, tzitzit, and mezuzah together preserve a person from sin, as Ecclesiastes promises, "A threefold cord is not quickly broken" (Ecclesiastes 4:12), and as the Psalm attests, "The angel of the Lord encamps about them that fear Him, and delivers them" (Psalms 34:7).
The Torah does not demand what a body cannot give. It asks for a sign, faithfully worn, and in that sign it reads the whole library of your soul.
(From the 1901 Hebraic Literature anthology, citing Yalkut Shimoni and Menachot 43b.)