"And if a man open a pit" — the Torah addresses the liability of someone who uncovers or creates an open pit in a public area. But the Mekhilta notices that the verse mentions only "opening" a pit. What about digging a new one? Is the digger also liable?
The Torah answers this in the very next phrase: "If a man dig a pit." Now both opening and digging are covered. But the Mekhilta asks: was the second phrase even necessary? If someone who merely opens an existing pit is liable, surely someone who digs a brand-new pit — doing more work to create more danger — should be liable too. The a fortiori argument seems obvious.
The Mekhilta explains why the Torah stated it explicitly anyway. There is a fundamental rule in Jewish law: punishment cannot be mandated by an a fortiori argument alone. Even when logic conclusively demonstrates that liability should exist, the Torah must state it directly before punishment can be imposed. You cannot punish someone based solely on "it stands to reason."
This principle protects defendants from judicial overreach. Judges cannot reason their way into imposing penalties that the Torah did not explicitly authorize. The Torah's mention of "digging" alongside "opening" was not redundant — it was constitutionally necessary. Without it, the logically obvious liability of the digger would have been unenforceable. Logic creates expectations. Only explicit text creates law.