As we have seen so far, the main function of the mezuzah is to protect the house from evil. Even to begin to understand the mechanism of this effect of the mezuzah, however, we must first delve into the concept of evil itself.

Overly simplistic logic might lead one to believe that if G‑d is by definition good, than evil must be a force opposing and independent of G‑d. Such a conclusion, however, would leads to a dichotomy utterly incompatible with monotheism. To believe in an independent evil, whether in the form of a god of evil, as in Zoroastrianism, or in the form of a rebellious angel (devil), as in Christianity, is always a form of polytheism and is utterly rejected in Jewish theology.

Judaism recognizes only one G‑d who is the source of the totality of existence, whether we perceive it as good or as evil. As the Prophet states, “I form light and create darkness, I make peace and create evil, I am G‑d, I do all these things.” (Isaiah 45:7). Evil is created ex nihilo just as is the rest of the creation. However, it is created not for its own sake, but only as an instrument of free choice and is tolerated to the extent that it serves its purpose. Of course, we are thus faced with another theological problem: how can evil emanate from G‑d who is all good? To resolve this difficulty, we must carefully define the concept of evil. Just as in the physical world there is no such substance as darkness but simply the absence of light, similarly, in the spiritual world, there is no reality that is evil, only the concealment of G‑dly light (emanation). We must be careful not to say the absence of G‑d, as that would impose a limitation on the Infinite and Absolute Creator. However, in order to allow for an existence of beings that would not be absorbed and nullified in the Source, G‑d chose to conceal and withdraw His light to create, so to say, a “vacuum” where created beings would feel their independent existence. This, in oversimplified form, is the fundamental concept of tzimtzum – concealment and contraction of the primordial Divine light – which is the cornerstone of Lurianic Kabbalah. It demonstrates how a monistic creation can lead to apparent dualism in the created world.

The absence of light, of course, creates possibility for darkness – or evil. It is the cosmic task of man to discover G‑d hiding, as it were, behind the veil of darkness.1 Evil, by definition, is that which covers and conceals the true source of existence, the Creator. The word used to represent evil in Kabbalistic terminology, klipah, literally means shell or husk. It is something that has no independent value or existence but serves only to conceal the true inner essence. Obviously, the possibility for evil was created for a purpose, to provide us with freedom of choice. Were there no outer shell concealing the true reality we would be forced to obey G‑d’s will, denied free choice and therefore the reward as well. We have said before that freedom of choice is possible only where there are alternative possibilities for good and evil. Thus evil serves a positive role, providing us with a mechanism for exercising free choice. Without it, we would be reduced to mere robots, so to speak.

Conversely, there is no evil without freedom of choice. A predator killing its prey for food cannot be accused of committing an act of evil since it has no choice in this matter. It was created by G‑d with this instinct and it does not possess any freedom of choice. Similarly, angels cannot be called good, because they, too, are denied any freedom of choice, and serve their Creator just because they were created for this purpose. Only man, possessing free will, can rise above angels or fall below animals depending upon the choices he makes. Thus, we see that without evil there is no free choice and without free choice there is no good or evil. Evil therefore allows good to exist in the same sense that the ray of light can be seen only in a cloudy sky.

Once we understand that evil must exist, and that it serves a positive role in the scheme of creation, we are confronted with another problem: if evil is the absence of G‑dly light, what gives the “husks” energy? What sustains their existence? Of course, the same Creator who gives life to everything enlivens the klipoth (the husks). However, if the domain of holiness receives G‑d’s sustenance in abundance, the domain of evil, called in Kabbalah sitra achra, the “other side”, must feed itself on the “leftovers” as it is merely tolerated. G‑d allows a minute amount of life-giving energy to trickle down to the “other side” in order to maintain its existence. Too much of such energy kills it completely, as the Sages of the Kabbalah put it: “bright light blinds the eyes of the evil forces.” The intellect, particularly wisdom, called in kabbalistic terminology Chokhmah, is the bright light that disperses darkness. So the evil must always remain in darkness, feeding itself on what leaks through small “holes” in the domain of holiness. Therefore, in the Kabbalah, a hole or an opening is called “ra”, evil, because it allows small vestiges of holiness to leak, providing the evil with its life force.

Now we can understand how the mezuzah protects the house. A Jewish home, which is a miniature Temple, is a vessel of holiness. A door is an opening to an outside and strange world, to the “other side,” thus it is called evil. The forces of evil always abide by the door, as the Zohar informs us, because that is where they receive their nourishment. The mezuzah, which contains the wisdom of absolute monotheism expressed in the declaration “Hear O Israel, the Lord is our G‑d, the Lord is One,” is the ray of bright light which blinds the evil forces, denying them the right of entry and dispersing them. This is the mystery of the protective powers of mezuzah.