Actualism

In contemporary Analytic philosophy, actualism is a position on the ontological status of possible worlds; the actualist holds that only the actual world and its inhabitants can properly be said to exist. In this it is opposed by possibilism, which holds that possible worlds other than our own exist (in some sense) just as much as the actual world does. Or, more precisely, the possibilist holds that possible worlds other than the actual world exist and that what it means to say that other possible worlds exist cannot be reduced to, or analyzed in terms of, the existence of anything in the actual world. Actualists need not have any objections to the possible worlds interpretation of modal logic (most of them are happy to make use of it freely); but to remain an actualist they do have to hold that "possible worlds" are either (1) an eliminable metaphor, or else (2) reducible to some entities or sets of entities that actually exist.

An example: I could go to the movies today, and I could decide to stay home. That contingency is usually taken to mean that there is a possible world in which I go to the movies, and that there is another possible world in which I don't. Now, only one of these two worlds is the actual world, and which one it will be determined by what I actually end up doing. The possibilist argues that these apparent existential claims (that there are possible worlds of various sorts) ought to be taken more or less at face value: as stating the existence of two worlds, only one of which (at the most) can be the actual one. Hence, they argue, there are innumerably many possible worlds other than our own, which exist just as much as ours does.

Most actualists will be happy to grant the interpretation of "I could go to the movies, or I could stay home" in terms of two distinct possible worlds. 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. The actualist argues, instead, that when we claim "possible worlds" exist we are making claims that things exist in our own actual world which can serve as possible worlds for the interpretation of modal claims: that many ways the world could be (actually) exist, but not that any worlds which are those ways exist other than the actual, booming, buzzing world around us.

This leaves open the question, of course, of what an actually existing "way the world could be" is; and on this question actualists are divided. One of the most popular solutions is to claim, as William Lycan and Robert Adams do, that "possible worlds" talk can be reduced to logical relations amongst consistent and maximally complete sets of propositions. "Consistent" here means that none of its propositions contradict one another (if they did, it would not be a possible description of the world); "maximally complete" means that the set covers every feature of the world. (More precisely: a set of propositions is "maximally complete" if, for any meaningful proposition P, P is either an element of the set, or the negation of an element of the set, or entailed by the conjunction of one or more elements of the set, or the negation of a proposition entailed by the conjunction of one or more elements of the set). Here the "possible world" which is said to be actual is actual in virtue of all its elements being true of the world around us.

Another common actualist account, advanced in different forms by Alvin Plantinga and