The ninth commandment — "You shall not testify against your neighbor false testimony" — is more than a prohibition. It is the foundation of an entire legal system built on the reliability of witnesses. The Mekhilta unpacks this commandment by connecting it to one of the Torah's most dramatic legal procedures: the law of the scheming witness.
In (Deuteronomy 19:19), the Torah prescribes the punishment for a witness caught lying: "And you shall do to him as he schemed to do against his brother." Whatever the false witness intended to inflict on the accused — whether financial ruin, corporal punishment, or even death — that same penalty falls on the liar instead. The punishment mirrors the crime that was attempted, not the crime that was committed.
But the Mekhilta raises a crucial question. The verse in Deuteronomy tells us the punishment for false testimony. Where, then, is the exhortation — the warning not to do it in the first place? The answer comes from the commandment itself: "You shall not testify against your neighbor false testimony." This is the warning. The commandment at Sinai serves as the advance notice, and the law in Deuteronomy provides the enforcement.
This distinction between exhortation and punishment is fundamental to rabbinic legal reasoning. Jewish law requires that before any penalty can be applied, the offender must have been warned in advance. A punishment without a prior warning is unjust. By identifying the ninth commandment as the warning and the Deuteronomy verse as the penalty, the Mekhilta ensures that the law of false witnesses meets this essential standard of fairness.