An important, but significantly different notion of possibilism known as modal realism was developed by the philosopher David Lewis.
Problematic expressions include names of fictional characters, definite descriptions and intentional attitude reports.
Popular candidates for this role within an actualist ontology include possible worlds conceived as abstract objects, essences and dispositions.
Actualism and possibilism in ethics are two different theories about how future choices affect what the agent should presently do.
Actualists assert that it is only relevant what the agent would actually do later for assessing the normative status of an alternative.
But they argue that the possibilist goes wrong in taking this as a sign that there exist other worlds that are just like ours, except for the fact that we are not actually in them.
Thus, the actualist interpretation of "◊p" sees the modality (i.e., "the way" in which it is true) as being de dicto and not entailing any ontological commitment.
[4] From a modal realist's point of view, such as Lewis', the proposition "◊p" means that p obtains in at least one other, distinct world that is as real as the one we are in.
One of the most popular solutions is to claim, as William Lycan and Adams do, that "possible worlds" talk can be reduced to logical relations amongst consistent and maximally complete sets of propositions.
Another common actualist account, advanced in different forms by Alvin Plantinga and David Armstrong, views "possible worlds" not as descriptions of how the world might be (through a very large set of statements) but rather as a maximally complete state of affairs that covers every state of affairs which might obtain or not obtain.
The disagreements between these three views touch many areas in philosophy, including the semantics of natural language and the problem of intentionality.
This is due to the fact that various expressions commonly used in natural language seem to refer to merely possible and in some cases even impossible objects.
[8] Since actualists deny the existence of such objects, it would seem that they are committed to the view that these expressions don't refer to anything and are therefore meaningless.
Similar cases come from definite descriptions that fail to refer, like "the present king of France".
Intentional attitude reports about non-actual entities are another type of problematic cases, for example "Peter likes Superman".
[16][17] A well-known account relies on the notion of possible worlds, conceived as actual abstract objects, for example as maximal consistent sets of propositions or of states of affairs.
This account relies heavily on a logical notion of modality, since possibility and necessity are defined in terms of consistency.
[17] Another account attempts to ground modal truths in the dispositions or powers of actually existing entities.
[23] A theistic version of this account has been proposed in order to solve these problems: God's power is the truthmaker for modal truths.
The adherents of these positions disagree on which part of the actual world is the foundation supporting modal truths.
[25] Hardcore actualists, on the other hand, assert that modal truths are grounded in the concrete constituents of the actual world,[26] for example in essences or in dispositions.
Actualism and possibilism in ethics are, in contrast to the main part of this article, not concerned with metaphysical claims.
But possibilism has to face the objection that in some cases it sanctions and even recommends what actually leads to the worst outcome.
[28][33] Douglas W. Portmore has suggested that these and other problems of actualism and possibilism can be avoided by constraining what counts as a genuine alternative for the agent.