The Head vs. The Hand

The Rambam writes in the header to the laws of tefillin that the mitzvah of the shel rosh is to wear it on one’s head.  The mitzvah of the shel yad is to tie it one one’s hand.  The Rogatchover (Laws of Tefillin 3:4) proves in great detail that there is a major difference between these two mitzvot.  The mitzvah of the shel rosh is a mitzvah in the cheftzah, that the tefillin should be placed on the head.  However, by the shel yad, the mitzvah is the act of tying the tefillin on.  This difference can already be seen in the possuk (Devarim 6:8) וקשרתם לאות וכו והיו לטטפת בין עיניך, regarding the shel yad the focus is on the action of tying but regarding shel rosh the emphasis is the tefillin being on the person’s head.  This difference is reflected in the beracha as well.  The shel yad beracha focuses on the man putting the tefillin on as opposed to the beracha on the shel rosh which is on the cheftza of the tefillin.  Now we can understand why the Rambam counts tefillin as two mitzvot, mitzvah 12-13.  The Ramban asks why is it different than the white strings and the techeles string(s) of tzitzis, which is counted as one mitzvah?  The answer is that the different strings of tzitzis make up one cheftzah, they are “ענין אחד“, but the two types of tefillin are חלוק בגדרם (Likutay Sichos volume 39.) 

Why is there this difference between the two boxes of tefillin?  The Rebbe (ibid.) explains that the tefillin shel yad is משעבד הלב, the average Jew doesn’t have control of his heart (Tanyeh Ch. 17,) he must tie his emotions down, conquer his תוואות  so that he doesn’t bring them forth into action.  On the other hand, שיעבוד המוח is within everyone’s control (ibid.,) therefore the mitzvah is to have the tefillin on the head, to have one’s brain constantly in tune with Godliness.