Rabbi Nathan analyzed the Torah's laws about lethal weapons with a precise analogy: stone is compared to fist, and fist is compared to stone. This mutual comparison, drawn from the Torah's language about fatal blows, establishes two critical principles for determining when a killing is legally classified as murder.
The Torah mentions both stone and fist as potential murder weapons. From the comparison of stone to fist, we learn that just as a fist has the potential to kill — a powerful enough punch to the right spot can be lethal — so too, the stone in question must have the potential to kill. A pebble does not count. The stone must be large enough, heavy enough, and solid enough to inflict a fatal blow. If it lacks killing potential, the striker cannot be convicted of murder regardless of the outcome.
From the comparison of fist to stone, we learn something different: specificity. A fist is inherently specific — it belongs to the attacker, it is identifiable, it can be examined. Similarly, the stone must be specific and identifiable. If the fatal stone was thrown from a pile and cannot be distinguished from the other stones in the pile, the attacker is not liable for capital punishment.
This second requirement is remarkable. Even if someone threw a stone from a pile and killed another person, and even if every stone in the pile was large enough to kill, the inability to identify the specific stone that struck the fatal blow creates reasonable doubt. And in capital cases, the rabbis insisted that doubt must favor the accused.
Rabbi Nathan's analogy builds a legal standard that is simultaneously rigorous in defining murder and protective of the accused — a hallmark of rabbinic jurisprudence.