Here are some brief notes on Plantinga’s system of modal realism. I have not found this to be easy territory. Sources: Essays in the Metaphysics of Modality, and Christopher Menzels’s SEP article on Actualism.
In place of possible worlds we have a full set of abstract maximal states of affairs. These “worlds”, along with the concrete world we know, are all part of existence. The difference between an abstract maximal state of affairs and our everyday world is that ours happens to be the maximal state of affairs which “obtains” -- this obtaining is analogous to a propostion being true. Note: in some of Plantinga’s writings, he says our world is the actual world, and the abstract ones are not actual, yet they exist. It’s easy for “those of us at home” to be confused when the words ‘actual’ and ‘exists’ are divorced! But leaving this aside, it is a primitive fact that one world – ours—is picked out as the concrete world. Lewis was extremely critical of this aspect – in his system actual is a merely indexical term: the actual world is the world I happen to be in, but otherwise the worlds exist on even-handed terms relative to each other.
Another key aspect for Plantinga is that for any object/individual there exists an individual essence. This is unlike Lewis’ counterpart theory, where I have counterparts in other worlds, and there is no dogma saying precisely when it is that the counterpart’s properties differ enough from mine that it is no longer worthy of being called my counterpart. The postulate of individual essences also seems a brute primitive. It serves to replace possible individuals in modal statements with something ‘actual’. But it’s not an actual individual – it is an (platonic) essence. The notion of individual essence seems to me a way to avoid the question of what it is which makes a bundle of properties an individual. (Some technical objections to this part of the system are also summarized in Menzel’s article.)
Plantinga takes seriously that we need to have truthmakers for modal truths (which I like). He also thinks there are should be no non-existent objects or mere possibilia as truthmakers, so everything needs to be explained via things which exist. To make it work, he needs a panoply of abstract entities to cover all the angles. I don’t have a problem with this in and of itself, since I am open-minded regarding the existence of abstract objects. The 2 issues I have are those mentioned above: first, the way our world gets picked out of the set of abstract worlds is a primitive; and the postulate of individual essences (as distinct from individuals themselves) is another feature I find somewhat gratuitous.