The Torah states in (Exodus 20:22): "And when you make an altar of stones unto Me." The Mekhilta zeroes in on the Hebrew word "im" — which can mean either "when" or "if" — and asks a critical question: is building a stone altar mandatory or merely optional?
The word "im" is ambiguous. In many contexts it means "if," implying a conditional, voluntary action. If the Torah is saying "if you happen to build a stone altar," then the commandment is optional — build one if you choose, but you are under no obligation. This reading would make the stone altar a matter of personal piety rather than binding law.
The Mekhilta rejects this reading decisively. The proof comes from a parallel verse in (Deuteronomy 27:6): "Of whole stones shall you build the altar of your God." This verse uses the imperative — "you shall build." There is no ambiguity, no conditional language, no "if." The command is direct and absolute.
By reading the two verses together, the sages established that the word "im" in Exodus must be understood as "when," not "if." The altar of whole, uncut stones is mandatory. Israel is obligated to build it, not merely invited to consider it.
This interpretive method — resolving an ambiguous word in one verse by consulting a clearer parallel verse elsewhere in the Torah — is one of the foundational techniques of rabbinic legal reasoning. A single word can swing the meaning of an entire commandment, and the Torah's internal cross-references provide the key to unlocking its true intent.