72. It is a commandment that man should fix a Mezuzah at his door, so that everyone will be guarded by the Holy One, blessed be He, upon leaving THE HOUSE and entering THE HOUSE. This is the secret of, "Hashem shall preserve your going out and your coming in from this time forth, and forevermore" (Tehilim 121:8). For the secret of the Mezuzah always rests at the door, which is the entrance to the higher, BEING MALCHUT, THE ENTRANCE TO ZEIR ANPIN, ACCORDING TO THE SECRET OF, "THIS IS THE GATE OF HASHEM" (TEHILIM 118:20). This grade is called guardian, so one is guarded.
73. For man is not guarded except by the Holy One, blessed be He, who constantly guards and is present at the door TO THE HOUSE, while one is inside THE HOUSE. Another REASON FOR THE MEZUZAH is that one must never forget the memory of the Holy One, blessed be He. This REASON is like the Tzitzit, as said, "that you may look upon it, and remember" (Bemidbar 15:39). When one sees that reminder, he is reminded to do his Master's bidding. The secret of Faith is that the Mezuzah includes a male and a female together.
74. In the book of Solomon IT IS WRITTEN: near the entrance, against the two grades, comes a certain demon that has permission to harm. It stands at the left side OF THE DOOR. Man lifts up his eyes, sees the secret of his master's Name IN THE MEZUZAH AT THE RIGHT SIDE OF THE DOOR and remembers it, and THE DEMON cannot harm. You may say that if this is so, IT IS TRUE WHEN ONE COMES INTO THE HOUSE. BUT when leaving the door OF THE HOUSE outside, the demon TO THE LEFT SIDE OF THE DOOR is at the right side OF THE MAN and the Mezuzah to his left. How is man guarded then, if THE MEZUZAH is to his left?
75. AND HE ANSWERS, All that the Holy One, blessed be He, does, follows its own kind. There are two grades to man, one to his right and one to his left. The one to the right is called the Good Inclination and the one to the left is called the Evil Inclination. When one goes out of his house's door, that demon lifts up its eyes and sees the Evil Inclination dwelling at the left side. It is attracted to that side WHERE THE EVIL INCLINATION ABIDES, NAMELY THE LEFT, and is removed from the right. Then at the LEFT side rests his master's Name, NAMELY THE MEZUZAH, and it cannot approach him to cause him harm and that man goes out and is saved from it. Upon coming INTO THE HOUSE the Holy Name IN THE MEZUZAH is to his right, AND THUS THE RIGHT OVERPOWERS THE LEFT, and it cannot prosecute him.
76. One must therefore be careful not to soil the door to his house with filth and refuse, or spill dirty water FOR TWO REASONS; the one is not to desecrate his Master's Name IN THE MEZUZAH and the other is that then that fiend, NAMELY THE DEMON, has permission to cause harm. For that reason man must be careful about it and take care not to push away his Master's Name from the door to his house.
77. When man fixes a Mezuzah at his door, when he enters his house, the Evil Inclination and the demon guard him in spite of themselves and say, "this is the gate of Hashem, into which the righteous shall enter" (Tehilim 118:20). When there is no Mezuzah at a man's entrance, the Evil Inclination and that demon GROW STRONG AND come together and place their hands on his head when he enters and begin to say, Woe to so-and-so who went out of his Master's domain. From that time ON he is not guarded and there is no one to guard him. May the Merciful One save us.