DEUT276 Put tefillin on one's arm. Owing to the fact that man has a physical body, he is naturally attracted to physical pleasures. Unless man's physical component is held back by his soul, he will fall into sin, for his earthly side is always rushing to satisfy its every urge and lust. Man's soul, however, is at a disadvantage, for it dwells within man's physical body and is confined within earthly borders--very far from the soul’s realm, which is in the heavens. Far from its home, the soul is unable to subdue the body's urges and they threatened to overpower her. For this reason the soul needs protection from her corrupt earthly neighbor, lest man's physical component kill her, for she is trapped in his territory and is under his sway. Hashem wants His holy nation to have merits, so He commands us to post a number of strong guards around our soul--specifically to never cease speaking words of Torah, day and night; to place tzitzis on one's garments; to attach a mezuzah at one’s doorways; and to wear tefillin on one's arm and head. All these are reminders so that we abstain from sin and do not follow after our eyes and our lusting heart.