Eruvin 4 - The statement of Rav
Rav said, "There are three laws that are never mentioned in the Torah, but instead God taught them to Moses on Sinai." They thus have no written support, but were transmitted from teacher to student.
Firstly, amounts to which Torah laws apply. Now, how could that be, when we know that all such amounts are described in the Torah in terms of fruit of the Land of Israel? For example, to become impure in a house afflicted with tsaraat (spiritual impurity of the owner resulting in spots on the walls of his house), one have to stay there for enough time needed to eat half a loaf of bread, and we mean wheat bread, not barley. So we have wheat. Next: a bone fragment, to transmit impurity, needs to be at least the size of a barley kernel. Thus we have barley, and so on. Rav will answer that the Torah did not give the exact limits anywhere, it is the Sages who gave the limits, and they based their opinions on the fruit of Israel.
Secondly, the laws of the mikveh. Here too, how could this be, when the Torah described the mikveh! - True, but it did not give such details, for example, as that one hair makes a knot that prevents water, and makes the immersion invalid, while three hairs can never be tied in a strong enough knot.
Thirdly, the laws of partitions - that a fence ten handbreadths high separates a room into two legally separate areas. But we did learn this from the construction of the Temple and the height of the cherubim! Rav answers that the Torah did not go into such particulars as the fact that objects that are less than three hand-breadths away can be considered connected by an uninterrupted partition.
Art: Jean-Francois Raffaelli - Man Having Just Painted His Fence
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