Unlike Lewis’s Genuine Modal Realism, EMR has varied tenets and thus I will only outline some of the differences to GMR.
In contrast to Lewis’s GMR, EMR theories do not take other possible worlds to be concretely existing (the main reason for not wanting to accept Lewis’s theory), they consider them abstract. If we don’t have to say such strange creatures really exist, but can have all the benefits to modality that a possible worlds theory creates, then so much the better. But can the Ersatzist do it?
One Ersatz view is that possible worlds are sets of representations that consist of existing things that have been recombined, like we do when we create new and fantastical monsters by slicing up animals and switching parts around in our minds. The world is viewed like a huge book consisting of sets of propositions (‘there are black crows’ – etc), and any possible worlds are variations on this set of propositions (storybooks that paint the world in a different way). For example, they may lack the ‘there are black crows’ proposition and so there would be no black crows. In this sense they merely represent the actual world and exist as abstracts, not in a concrete (physical) sense that Lewis’s GMR suggests. There is one world (this world) that is actualised, and all others are unactualised.
One immediate thought you might perhaps have is:
‘How can we know any more surely that there aren’t other concrete worlds than we can know that there are?’
It does seem that the Ersatzist is taking things too far and denying things we don’t want to deny. The Ersatzist says there are abstract worlds, but why deny the possibility of concrete ones we do not know about? Agnosticism (not knowing) seems closest to common sense here.
It is also important to note that a great many of the problems with Genuine Modal Realism carry over to Ersatz Modal Realism, and this tends to be Lewis’s strategy to defending GMR. For example, how can we know of these abstract possible worlds? What can we say of counterparts or transworld identity in these cases? Surely a counterpart of the same kind of us (concrete) is more parsimonious than some abstract being? Furthermore, depending on the variety of EMR, most leave a lot to be explained. For example, attempting to explain possible worlds in terms of propositions, but fail to tell us about the nature of these propositions or taking terms modal concepts as primitive and as such cannot give a completely reductive account of modal logic.
I confess that time has run short and I will have to close the book on possible worlds and their goings on. I have done the Ersatzist a grave injustice in not fleshing out the various theories on offer, and as always in Philosophy, there is much debate. I advise reading further in alternatives to Genuine Modal Realism to get a more balanced account of the topic.
Resources: Handouts from my lecturer, Dr. John Divers (writer of ‘Possible Worlds’ (2002)), ‘Realism and Anti-Realism’ (2007) by Stuart Brock and Edwin Mares, ‘Modality’ (2003) by Joseph Melia and ‘On The Plurality of Worlds’ (1986) by David Lewis.