(Exodus 20:22) instructs: "Do not build them gazith." The Mekhilta explains that "gazith" means "gezuzoth" — hewn stones, specifically stones upon which iron tools have been used. Any stone that has been shaped by iron is disqualified from use in building the altar.
Rabbi Nathan then raised a practical question. Suppose someone built two stones into the altar that had been touched by iron, while the rest of the altar was constructed properly from unhewn stones. Would the entire altar be invalidated, or only the two offending stones?
The answer comes from the Torah's precise wording. "Do not build them gazith" — the "them" refers to the individual stones, not to the altar as a whole. The hewn stones themselves are unfit, but they do not contaminate the entire structure. Remove the offending stones, and the altar remains kosher.
This distinction matters enormously in practice. If any contact with iron invalidated an entire altar, then a single mistake during construction would require demolishing and rebuilding the whole thing. Rabbi Nathan's reading saved the altar from this all-or-nothing fragility. The prohibition targets the specific stones that were hewn, not the structure they were placed in. This reflects a broader principle in Jewish law: contamination and disqualification are usually localized to the offending element rather than spreading to everything it touches, unless Scripture explicitly states otherwise.