The Rambam explains that the affliction of tzara'as was decreed to occur in three incremental phases upon a speaker of l'shon ha-ra (evil speech): Tzara'as first strikes the home of the offender; if he continues in his malicious ways, then the disease comes upon his clothing; if the offender is unaffected by this, then tzara'as strikes his very body. (Hil. Tzara'as 16:10)
The obvious question is why, according to the Rambam, does the Torah present the three stages of tzara'as in the reverse order. The Torah first details the laws of tzara'as of the body, then of clothing, then of houses. Why is the disease not presented in the order of its progression?
The answer to this question may be found by turning back to the beginning of Parshas Tazria, where the Torah declares that a woman who gives birth becomes temi'ah - spiritually/ritually impure. (See elaboration on this theme in d'var Torah on Parshas Tazria 5763 by clicking Archive link below.) The notion of tumah - spiritual/ritual impurity - of a childbearing woman is a declaration of the vulnerability of Man. Man (male and female) is designated by God as slightly below the level of the angels, endowed with speech and creative traits that distinguish him from the rest of Creation. Man is likened to God Himself in terms of Man's charge to rule the universe and conceptualize its workings. Thus, declares God, the very same same Man, acting in the role of a creator upon giving birth, must be reminded of his finite, earthly essence. That is why tumah falls upon a woman at childbirth, as the highly-gifted, God-like being called Man must be reminded that he is from the earth and is prone to descend to an earthy, unholy existence, just like the basest of animal life. Man can use his unique powers to fulfill his potential, or he can totally corrupt it.
So, too, Man is elevated above the rest of physical and organic life by his speech; the Targum defines Man as a "Ruach Memalela" - a "Speech-Endowed Spirit". Thus, the Torah begins its treatise on human purity and impurity by declaring that the traits which most distinguish Man from the rest of Creation can themselves become his very downfall, as Man is vulnerable and may not live up to his potential. Man can use his creative powers and his speech for Torah and for uplifting the universe, or he can abuse these gifts and debase himself and his surroundings.
The two qualities of Man by which he is likened to God - creativity and speech - are thus presented at the beginning of Parshas Tazria to proclaim Man's limitations and vulnerability, for the perversion of these Godly traits is the antithesis of holiness. This is why the Torah begins with tzara'as of people rather than with that of houses.
From a broader perspective, one can view Sefer Vayikra on two levels. On the one hand, Vayikra depicts the manifestation of the Shechinah - the Divine Presence - in the Mishkan, and the Avodah - Sacrificial Service - mandated by this phenomenon. On the other hand, the desire of God to be close to Man and enable Man to approach Him - entering the innermost sanctums of the Mishkan and serving therein on a continual basis - is a tribute to Man's special, privileged and elevated status. Vayikra provides a glimpse of God's holiness, as manifest to the physical universe via the Shechinah, and it also emphasizes the unique, privileged status of Man, who was gifted to encounter God in an unparalleled fashion in this world.
It is thus understood why the forms of tumah which relate most to Man's essence are emphasized first, with the Torah progressively moving away to more distant and ancillary forms of tumah (upon clothing and the house), as Vayikra addresses the central role of Man and his ability to be holy or the reverse; thus, tumah relating to Man's unique essence is presented first.
Man is charged to elevate himself and his world, and he is gifted for this purpose. Abuse of these gifts lowers Man beneath the level of the animal, while proper use of Man's gifts enables him to fulfill his Godly mission.
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