10:10 When putting on tefillin, one must think about how God commanded us to wear these boxes that contain four sections of the Torah that describe His Oneness and how He took us out from Egypt. The arm tefillin are placed adjacent to the heart and the head tefillin adjacent to the brain so that we always remember the miracles that God worked for us, demonstrating not only His Oneness but also His might and His dominion over all creation. This should remind us to serve Him because our consciousness is in our brains and our desires are in our hearts. (Obviously this is not speaking medically.) We should remember God and subjugate our passions as per Number 15:39, “and you should not pursue after the desires of your hearts and your eyes.” For this reason, tefillin are placed between the eyes. 10:11 The tefillin of the hand and those of the head are two separate mitzvos; the non-performance of one has no effect on the other. Therefore, if a person only has half of a pair of tefillin, or for some reason cannot put one on, he should put on whatever he can. If he puts on the arm tefillin only, he only says the bracha of l’haniach tefillin; if he only puts on the head tefillin, he says both l’haniach tefillin and al mitzvas tefillin (plus Baruch Shem, as per usual).