The sages of the Talmud take it as a given that tefillin are put on the left arm (or the right arm of a lefty) and offer several reasons:

1. “Hand”=Left Hand

According to one tradition, whenever Scripture uses the word yad (“hand”) without defining which one, it refers to the left hand. For example, the prophet Isaiah states, “Even My hand laid the foundation of the earth, and My right hand measured the heavens with handbreadths . . .1” Notice that there is the “hand” and there is the “right hand,” implying that the unidentified hand is the left hand.

Thus, when the Torah tells us to put tefillin on our “hand,” the implication is the left hand.2

2. Bind With the Writing Hand

We read, “And you shall bind them for a sign upon your hand . . . And you shall inscribe them upon the doorposts of your house . . .”3 The juxtaposition of binding and writing teaches is us that we should bind the tefillin with the same hand used to write the parchments contained therein. Just as one writes tefillin scrolls with his right hand,4 so should the binding be done with the right hand. This effectively means that tefillin must be worn on the left hand.5

3. The Weak Arm

The third (and most widely known) reason is that in Exodus,6 when telling us to bind tefillin on the arm, the word yadcha (“your hand”) has an extra hei at the end (ידכה). The sages explain that the elongated word can thus be read as two words (יד כה), meaning “the weak arm.” Thus, tefillin are bound on the left, since it is generally the weaker arm.7

4. Corresponding to the Heart

The Talmud tells us that the hand tefillin need to be placed “facing the heart.”8 Although some interpret this as merely telling us that the tefillin need to be placed on the bicep, which is level with the heart, others explain this to be an additional reason for placing the tefillin on the left arm, which is nearer to the heart.9

Although there are various reasons for putting the tefillin on the left arm, out of all of them, we are meant to bear this one in mind while putting on tefillin. In the words of Rabbi Schneur Zalman of Liadi in his siddur:

“When one puts on tefillin, he should bear in mind that G‑d commanded us to write on the parchment contained in the tefillin the four specific biblical passages which mention His Unity and the Exodus from Egypt, in order that we remember the miracles and wonders He performed for us. They indicate His Unity and demonstrate that He has the power and dominion over those above and below, to do with them as He wishes. And He has enjoined us to place the tefillin on the arm adjacent to the heart, and on the head over the brain so that we submit our soul, which is in the brain, as well as the desires and thoughts of our heart, to His service. Thus, by putting on the tefillin, one will be mindful of the Creator and restrict his pleasures.”10

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The rabbis offer additional reasons for strapping tefillin on the left hand:

The Holier Hand

Rabbi Hezekiah ben Manoah (13th century), known for his commentary on the Torah called Chizkuni, explains that since the right hand is the more active hand, used for mundane, menial tasks, it isn’t fitting that the tefillin, which contain G‑d’s holy name, be strapped to it. Instead, we place them on the left hand, which isn’t used as often.11

Closer to the Person

Rabbi Yehuda Lowe (1525-1609), known as the Maharal of Prague , explains that we are commanded regarding tefillin, “And these words, which I command you this day, shall be upon your heart.”12 Since a person’s right hand is stronger, it is more deft, and therefore it’s in constant use and “further away” from the person. So we put the tefillin on the left hand, which is usually kept “closer” to the person.13

Left-Handed Punishments

Rabbi Chaim ibn Attar (1696-1743), known as the Ohr Hachaim, points out that the Torah tells us to put on tefillin to remind us that “with a mighty arm the L‑rd took [us] out of Egypt.”14 The mystics explain that the “mighty arm” used to punish the Egyptians refers to the attribute of severity and judgment, which corresponds to the left side. Thus, tefillin are worn on the left arm, reflecting G‑d’s “left arm,” which performed the miracles in Egypt. (Wondering what it means for G‑d to have an arm? Read up on it here.)15

Overcoming Our Physical Desires

Rabbi Shlomo Ephraim Luntschitz (1550-1619), known as the Kli Yakar, explains that the right side represents physical desires and strength, while the left, weaker side represents the (more restrained) spiritual and intellectual strength. Wearing tefillin on the left side demonstrates that it is the spiritual and intellectual strength of the heart that “tames” the physical desires (and causes the left side to be physically weaker).16

Only With G‑d’s Help

The Kli Yakar offers an alternative explanation: putting tefillin on our weaker hand symbolizes that without the help of G‑d, we are weak and don’t have power to accomplish anything on our own.17 Keeping this in mind as we start every day gives us the strength and faith to stay on the straight path throughout the day, knowing that ultimately all is in the hands of G‑d.

Transforming Negative

The fifth Lubavitcher Rebbe, Rabbi Sholom Dovber, known as the Rebbe Rashab, explains18 that through putting the tefillin on the left arm next to the heart, one subjugates his animal soul to the G‑dly soul, transforming the negative desires into positive. Thus, bit by bit, we transform negativity into positivity, until we reach the state in which, as the mystics put it, “there will only be a right (i.e., positive) side.” This will come to fruition and be revealed in the messianic era—may it be speedily in our days!