Legal scholars distinguish between chattel and fixtures. Chattel refers to movable property like clothing and books; anything that can be removed without doing damage to physical property. Fixtures, on the other hand, are annexed to the realty, that is, permanently attached to the physical structure, so that removal would be impossible without causing damage. It is unclear whether what we call a “mezuzah” (Deuteronomy 11:20) is chattel or fixture.
Definitionally, a mezuzah can be removed without causing physical damage to the doorpost. Depending how the mezuzah case is affixed removal can be achieved by simple unscrewing. Even in circumstances where the mezuzah case is attached by glue, the residue following removal is probably insufficient to call it significant damage. Halakhically, however, the mezuzah is treated like a fixture. For instance, when a Jew sells his house to another Jew, he is forbidden to remove the mezuzah (cf. Babylonian Talmud, Bava Metzia 101a-b).
Treating the mezuzah as a fixture has two significant emotional consequences. First, the mezuzah serves as an emotional anchor in an evanescent world. As Rabbi Yissachar Frand frames it, “the mezuzah is a silent witness to the ebb and flow of history.” Second, as Maimonides (Laws of Mezuzah 6:13) writes: “…whenever one enters or leaves [the house] he encounters the oneness of God, the Name of the Almighty, and will recall the love for Him and will awaken from his slumber and preoccupation with the vanities of the time. He will realize that there is nothing that remains forever and ever except the awareness of [God,] the Rock of the world.” In other words, the permanence of the mezuzah parallels the permanence of God.
The permanent mezuzah parallels the eternal God.