Discipline: Philosophy
Name often given to either or both of the identity of indiscernibles and the indiscernibility of identicals; called after German philosopher and mathematician Gottfried Wilhelm Leibniz (1646-1716).
Leibniz himself seems to have held explicitly only the first, and to have treated it sometimes as necessary and sometimes as contingent.
Source:
H G Alexander, ed.. The Leibniz-Clarke Correspondence (1956)