A person should touch his tefillin [from time to time] during the entire time he is wearing them, so that he will not divert his attention from them even for a single moment, for their holiness surpasses that of the tzitz. The tzitz has God’s name [written] upon it only once, while the head tefillin – and similarly, the arm tefillin – contain the name י-ה-ו-ה 21 times.