Musar for Menachot 86:10

We have been commanded to perform the above mentioned three commandments in order to achieve a degree of perfection, as our sages have pointed out that anyone who wears the fringes on his garment, the phylacteries on his head and arm, and whose door posts are adorned with mezzuzot, falls under the category of people concerning whom it has been said "that the threefold thread will not break easily" (Kohelet 4,12 quoted in Menachot 43). The ציציות surround a person's body from all four sides, and allude to its being whole. The מזוזה on his house symbolises that all is well with him financially. Whatever a person owns he keeps in his house and closes the door, allowing the presence of the מזוזה to act as his protection. This is why it is called מזוזה, i.e "it prevents it from being moved." It is man's demonstration that he believes that whatever he owns originates from G–d and has to be devoted to His service. The phylacteries on a man's head allude to his serving G–d with his mind, the ones on his arm are positioned opposite the heart, a source of his thought processes. Mental activity is the combined result of brain and heart. This alludes then to the wholeness of his Torah. I have already mentioned that these three areas of "wholeness," are the ones referred to earlier as חכם, גבור, עשיר.