God commanded the Israelites to "wash their garments" in preparation for receiving the Torah at Sinai (Exodus 19:10). The Mekhilta asks a follow-up question that the Torah itself does not address: did the people also require immersion — full-body submersion in water — or was washing their clothes sufficient?
The answer comes through a logical argument known as a kal vachomer, an a fortiori inference — one of the fundamental principles of rabbinic reasoning. The argument works by comparison: consider a case where washing of garments is not required, such as someone who touches a sheretz (a ritually impure creeping creature). Even in that lesser case, immersion of the body is still required for purification. If immersion is necessary when garment-washing is not, how much more so must immersion be necessary here, where garment-washing is explicitly commanded?
The logic is elegant. The Torah established two forms of purification — washing garments and immersing the body — and applied them to various situations with different levels of stringency. The Mekhilta observes a universal rule: there is no instance in the entire Torah where washing of garments is required without immersion also being required. The reverse is not true — immersion can be required without garment-washing — but whenever garments must be washed, the body must also be immersed.
This principle meant that the Israelites at Sinai underwent a complete purification process. They did not merely launder their clothing as an external preparation. They immersed their entire bodies, achieving a state of ritual purity appropriate for the most significant revelation in human history.