  Another option would be to take possible worlds as sets of spacetime points and think of each such set as representing the possibility that all and only the points in it are occupied (the view is proposed as an illustration in Cresswell 1972,   136).[24]  This assumes, as Cresswell notes, that all properties of things are determined by the properties of certain basic entities whose properties can all be expressed in terms of the spacetime points they occupy. If sets of spacetime points can be seen as concrete then this might be a way of taking possible worlds as concrete. This view derives from certain passages by Quine, where he develops the idea that every distribution of space points could be taken as a possible world momentary state (1969, 148). But to avoid certain difficulties (some having to do with ontological economy, others having to do with the notion of a point and the relativity of position), Quine proposes to bypass spacetime points and takes possible worlds as certain sets of number quadruples (Quine 1969, 151). To be nominalistically acceptable this account of possible worlds would need to be accompanied by a nominalistically acceptable treatment of sets and numbers.