The Torah says that if men quarrel and one strikes the other "with stone or fist" (Exodus 21:18), the striker is liable. Does this mean liability exists only for these two specific implements — a stone or a fist — and nothing else?
The Mekhilta rejects this narrow reading by turning to (Numbers 35:17): "And if with a hand-stone, whereby he can die, he strike him." This verse introduces a critical qualifier: the implement must have "the potential to kill." The focus is not on what the weapon is made of but on whether it could realistically cause death.
From this, the rabbis derived a two-part test for liability in assault cases. First, the implement must have the potential to kill — a pebble might not, but a large stone could. Second, the blow must land on a body part that is "critical to life" — a strike to the head is different from a strike to the hand.
Both conditions must be met simultaneously. Striking someone with a deadly weapon on a non-critical area, or striking a critical area with a harmless object, does not trigger the same level of liability. The Torah's mention of "stone or fist" was not meant to create an exhaustive list of weapons but to establish that the materiality of the implement matters. What counts is physics, not category: could this object, striking this body part, actually kill a person?