 Van Inwagen distinguished between Abstractionist and Concretist conceptions of possible worlds (see van Inwagen 1986: 185–6; see also Stalnaker 1986: 121). According to a Concretist conception of possible worlds such as David Lewis’s, a possible world is the maximal mereological sum of individuals that are spatio-temporally related and each such sum is spatio-temporally isolated from all others (see Lewis 1986: 69–70). There are no impossible worlds. The actual world is the world in which we are located.