 While all of the above no doubt express some important type of dependence relationship, there are also important differences between them. (1) may be understood as stating that a set {x, y, z} could not exist if its members, namely x, y, z, did not exist. One way to analyze the type of dependence in question is as rigid existential dependence, to be clarified in a moment. Another sense of dependence that could be at work in (1) is identity-dependence, but we will return to this later on. In (2), we seem to have in mind a more general kind of dependence: there could not be electricity, now or ever, if there were no electrons. So the existence of electricity depends on the existence of a very specific kind of particle, the electron. This second type of dependence is also existential, but to separate it from the rigid dependence in (1), we may call it generic existential dependence. (3) is a famous case borrowed from Fine (1994a, 1994b). Singleton Socrates, expressed by “{Socrates}”, is the set containing Socrates as its sole member. From (1) we see that the set itself plausibly depends on Socrates, but since the set necessarily exists when Socrates does, the dependence between them would seem to be symmetric. However, (3) captures the very plausible idea that it is not any part of the essence or identity of Socrates that it should belong to {Socrates}. So, (3) must refer to some form of essential dependence (or lack thereof in this case).