in pari causa

from Bouvier's Law Dictionary, Revised 6th Ed (1856)
IN PARI CAUSA. In an equal cause. It is a rule that when two persons have 
equal rights in relation to a particular thing, the party in possession is 
considered as having the better right: in pari causa possessor potior est. 
Dig. 50; 17, 128; 1 Bouv. Inst. n. 952. 
    

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