In general, the mishna says, “one takes an oath only concerning an item that is defined by size, by weight, or by number.” That is, the dispute must be about the quantity of a substance, not about the substance itself. But the question of what defines a substance, what makes a thing the thing it is, is one that always creates problems for the rabbis. Thus the Gemara in Shevuot 43a asks about a situation where the plaintiff claims a large candelabrum from the defendant, and the defendant admits only to possessing a small candelabrum.Earlier Daf Yomi columns are noted here and links.
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