 To understand this we need to appreciate that Armstrong’s notion of supervenience is non-standard: “an entity Q supervenes upon entity P if and only if it is impossible that P should exist and Q not exist” or, in other words that the existence of P entails the existence of Q. (Armstrong 1997: 11). He also holds a non-standard conception of ontological commitment, viz. that “What supervenes is no addition of being” (Armstrong 1997: 12, 2004: 23–4). Armstrong’s idea is (roughly) that to be a genuine addition to being is to be a net (indispensable) contributor to the schedule of truth-makers for all the truths. But supervenient entities are superfluous as truth-makers. If the existence of an R entails the existence of a certain S which in turn entails the truth of P, then R already makes P true so there is no need to include S in the inventory of truth-makers. It follows that supervenient entities, like S, are no addition of being (Lewis 1992: 202–3).