rule of the floating subtrahend
rule of the floating subtrahend. The common-law doctrine that a plaintiff whose damage was not caused entirely by the defendant must prove the amount of damage that is not attributable to the defendant (the subtrahend) or else recover nothing. • The reasoning behind the rule is that damage is an essential element of a tort claim, and the plaintiff has the burden of proof. If proved, the subtrahend is subtracted from the total damage to determine the plaintiff’s recovery.