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Actualism is the metaphysical thesis that only actually existing entities are real; possible but non-actual entities do not have being. Where modal realism posits a plurality of concrete possible worlds and their inhabitants as real, actualism analyzes modality in terms of what the actual world could have been or through abstract constructions, without populating ontology with non-actual entities.
You have already encountered modal realism — David Lewis's view that possible worlds are concrete, spatiotemporally isolated universes as real as our own, and that when we say "there could have been talking donkeys," we are literally asserting that such animals exist in some possible world. You've also worked with possible-worlds semantics, the formal machinery that uses worlds as the domain over which modal operators ("necessarily," "possibly") range. Actualism is the rival ontological commitment: only this world, the actual world, and its inhabitants genuinely exist. There are no concrete non-actual possible worlds populated with non-actual individuals — no flying donkeys, no merely possible persons who were never born.
The core challenge for actualism is explaining what modal statements are *about*. If "It is possible that there are talking donkeys" is true, and there are no non-actual talking donkeys, what makes it true? The actualist cannot point to a concrete entity. Two main strategies fill this gap. The first appeals to abstract possibilia — not concrete worlds but abstract objects like sets, propositions, or states of affairs that represent how things could have been. On this view, a possible world is a maximal consistent set of propositions or a complete way things could be, understood as an abstract structure rather than a parallel universe. The second strategy, associated with Robert Adams and others, appeals to essences and haecceities — abstract individual essences that an entity would have had if it had existed, without the entity itself existing non-actually.
The contrast with Lewisian modal realism is stark. For Lewis, the actuality predicate is indexical: "actual" just means "the world I'm in," the same way "here" means "the place I'm at." From any world, that world counts as actual. For the actualist, actuality is absolute — there is a privileged, genuinely existing world, and possibility is analyzed in terms of that world's features, not in terms of other equally real worlds. This makes actualism more parsimonious ontologically (no commitment to an infinity of concrete universes) but theoretically harder: the actualist must say what abstract constructions like "possible worlds" are, and how they represent modality without being the concrete things Lewis invokes.
The practical stakes become clear when you consider transworld identity — whether the same individual can exist in multiple possible worlds. Lewis solves this with counterpart theory: "you" appear in other worlds not as literally you, but as your counterparts who resemble you closely. The actualist who accepts possible-worlds talk while denying concrete worlds faces a different question: since there are no non-actual worlds for you to exist in, modal claims about you are claims about how the actual world could have been different — about which propositions involving your essence are compossible. Actualism thus shapes every downstream question about the logic of identity, essence, and necessity, and understanding it is essential for following the debates it enables.
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