Actualism, with respect to possible worlds, is the view that if there are any true statements in which there are said to be nonactual possible worlds, they must be reducible to statements in which the only things there are said to be are things which there are in the actual world and which are not identical with nonactual possibles. The actualist will not agree that there are nonactual possible worlds, if the notion of possible worlds is to be regarded as primitive. Possibilism, with respect to possible worlds, is the view that there are nonactual possible worlds and that the notion of a possible world is not to be analyzed in terms of actual things. … As we shall see, it may involve the difference between an absolute and a world-relative concept of truth. [Robert Merrihew Adams, "Theories of Actuality," in Michael J. Loux, ed., The possible and the Actual: Readings in the Metaphysics of Modality, 202-3]
(20) It is possible that there exists an object distinct from every actually existent object. [Michael J. Loux, "Introduction" in Loux, ed., 45]
What do the term "actualism" and "possiblism" mean when discussing the issue of modality? One might guess it is somehow related to realism and anti-realism as it comes to the nature of things, and that would be close. However, it is possible to be an actualist and an anti-realist concerning possible worlds (probably Plantinga?), and possible to be a possibilism and an extreme realist (e.g. David Lewis). So actualism does not mean realism, and possibilism does not mean anti-realism. What are they then?
As defined by Robert Adams, the difference between actualism and possibilism concerns the type (and the quantity) of things in any possible world, and that entire possible world as well. An actualist believes that all possible worlds can only be construed out of what is in the actual world. In other words, they deny statement 20 as stated by Michael Loux above. An actualist must also deny the extensionist sense of the statement:
It is possible for there to exist more things than actually exist.
In my understanding, actualism therefore thinks that all possible things, worlds, etc, must in some way exist in the actual world. The "furniture" to work on are only things that we can see in the actual world. For possibilism on the other thing, the sky's the limit. For both, it is of course possible to claim nonactual things. In actualism however, nonactual things can be taken to be mere words, and it is impossible to evaluate truth conditions for them, since nonactual things do not, well, exist.
It is more in line with commonsense to hold to the existence of possibilia, although actualists have many objections to possibilism. Thus, without taking a firm position on the issue, I hold to possiblism for now, noting that we can talk about truth values in possible worlds of fiction, as for example evaluating a scene from the Star Wards universe according to interal world coherence.
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