 Evidently, it would not do simply to replace the conditional connective in (ID-NEC) with a biconditional connective and thence attempt to turn it into a definition of identity-dependence, unless at the same time one could impose some suitable restriction on the kind of function involved. One possibility would be to exclude any function f which is not such that it is part of the essence of x that it is the f of y. But since it would appear strange to say that essences quite generally involve anything about functions, we might be better off with a definition using an appropriate yet to be specified place-holder relation that would do the same job instead, arriving at the following definition of identity-dependence: