 While counterpart theory was Lewis’s invention, it was also a product of its time, the 1960’s, when quantified modal logic was “in the air” and questions about essentialism and the so-called problem of  transworld identity  gained attention (e.g.  Quine 1953, Hintikka  1962).[7]  The counterpart-theoretic approach does, however, bear a striking similarity to certain views of Leibniz concerning essential and accidental  properties.[8]  (See Mondadori (1973) for a counterpart-theoretic interpretation of Leibniz.)