This week’s Torah reading of Tazri’a-M’tsora (Leviticus 12-15) is largely concerned with laws of purity and impurity relating to the disease of tsara’at, usually rendered as leprosy. Not only can this ailment infect people, but it can also afflict clothing and houses, in certain cases causing all vessels within a structure to contract ritual impurity. When a house appears infected, Scripture requires the owners to remove everything therein, and then ask a priest to inspect it, who will then either declare it pure or impure. Martha Himmelfarb examines the reasons for this step, and its significance for understanding the nature of Jewish law:
Once the process of determination begins, everything in the house— whether made of cloth, leather, hair, or metal—is subject to impurity. . . . In other words, to spare the person time, money, and anguish, the Torah permits—even encourages—the homeowners to clean out their home before the priest enters and evaluates it.
Once the priest recognizes the signs of leprosy, [however], shouldn’t the vessels and furniture be automatically impure, even if removed? After all, they were in an impure house! How does removing the vessels protect them from impurity? The legal loophole used here seems to reflect a role for human intention in law. Physical reality alone does not convey impurity. At least in regard to house leprosy, until the priest decides to close the house for a seven-day quarantine, it and the objects inside it remain pure.
Human intention and engagement play an important role in rabbinic law. For example, people must intend to fulfill the commandment to hear the sound of the shofar in order to fulfill the commandment. Just physically hearing the note of a ram’s horn is insufficient to make it count as a ritual act of shofar blowing. . . . Until the priest has declared the house quarantined because of leprosy, the house and its objects remain in a state of purity. Ritual reality is determined by human beings.
This kind of legal thinking, ubiquitous in rabbinic literature, is admittedly not common in the Torah. To the best of my knowledge, this leprosy passage is the only such instance. The passage thus marks the beginning of a process of legal thinking that would later be developed by the rabbis and become part and parcel of how Jewish law functions.