Torah Methodology #2 – Gezeira Shava
A gezeira shava is a tradition of a known rule applying to a new case based upon an identical word or phrase in both cases. For example, the Torah uses the word “b’moado” (“in its proper time”) when referring to the korban Pesach (Numbers 9:2) and when referring to the daily korban Tamid (Numbers 28:2). Just as the korban Tamid was brought “in its proper time” – even on Shabbos – so, too, the korban Pesach was to be offered “in its proper time” – that is, even on Shabbos. (This gezeira shava is put forth by the Talmud in tractate Pesachim on page 66a.)
Another example: We are told that a paid watchman who has had an object in his care stolen must take an oath that he didn’t lay his hands on what he was given to watch. The Talmud (Shevuos 47a) clarifies that the oath is only between the two parties, not between their heirs if one of the parties should die before the case is settled. The details of the oath are not told to us regarding an unpaid watchman but the Talmud applies them in that case as well, based upon a gezeira shava of the phrase “im lo shalach yado b’meleches r’eihu” (“that he did not lay hands on his fellow’s property”), which is found regarding a paid guardian in Exodus 22:10 and regarding an unpaid guardian in Exodus 22:7.
Unlike a kal v’chomer, which is a strictly logical argument that anyone can propose, a gezeira shava can only be put forth based upon a received tradition.