Halakhah for Zevachim 37:7

It is from the root of the commandment [that] since people are physical, they are necessarily drawn after their desires; for the nature of all physical things is to seek all which is comfortable for it and is pleasurable - just as a horse or like a mule who understand nothing - were it not that the soul with which God has graced us prevents us, to its ability, from sin. But as it resides in [the body's territory,] which is the earth, and is far removed from its [territory], which is the heavens, it cannot [vanquish] it and [the body] always exerts its strength over it. It therefore nonetheless requires many guards to protect it from its malevolent neighbor, lest it rise up upon it and kill it; as it is in its [territory] and under its hand. And the Omnipresent, blessed be He, wanted to give us - the holy nation - merit, and [so] He commanded us to set up mighty guards around [the soul]. And they are that we were commanded not to interrupt from [speaking] words of Torah from our mouths day and night; that we place four fringes on the four corners of our garments; a mezuzah on our doorpost; and the tefillin on our arm and on our head. And all of this is to remind us to avoid the crimes of our hands and that we not stray after our eyes or after the impulse of the thoughts of our hearts. Therefore, they, may their memory be blessed, said (Zevachim 19a) that the priests and the Levites are exempt from them when they performed the [Temple] service. And as the foundation of the tefillin is as we have mentioned, we are commanded about them (Menachot 36b) that we not remove our thoughts from them. Now, my son, also see how much more power our bodies have than our souls; for despite all this, it sometimes comes up and 'breaks our fence.' May God in His mercy aid us and protect us, amen.