Chisholm’s paper “Identity through Possible Worlds: Some Questions” (1967b) was the lead article in the first issue the new journal Noûs in 1967. In that article, Chisholm presented an argument that provoked a remarkable amount of discussion and debate. A variant of the argument was subsequently presented in Chisholm’s “Parts as Essential to Their Wholes”, which was Chisholm’s Presidential Address at the Metaphysical Society of America in 1973. This essay corresponds closely to Appendix B of [P&amp;O]. The puzzles presented in those papers have come to be known as versions of “Chisholm’s Paradox” (though it must be pointed out that the term ‘Chisholm’s Paradox’ is also widely taken to refer to the problem about contrary to duty imperatives that is discussed below in Section 11).