Modal metaphysics, transworld identity, and the modal ontological argument: are possible worlds abstract states of affairs that the actual world instantiates, do individuals retain numerical identity across them, and does the modal ontological argument show theistic belief rational?

Alvin Plantinga, answered from the texts and cited to the page.

The position in brief

Possible worlds are maximal, possible states of affairs: an abstract entity exists for every possible way the actual world could have been, and the actual world is the maximal state of affairs that obtains. Actualism holds: only what exists actually exists, full stop; there are no merely possible individuals floating in non-actual worlds. Haecceities — individual essences — exist necessarily, and they are how individuals 'show up' in worlds where they themselves do not exist: Plantinga's haecceity is necessarily the property of being identical to Plantinga, and it exists in every world even where Plantinga does not. Transworld identity is preserved for individuals — Plantinga in one possible world is numerically the same individual as Plantinga in another.1234

The modal ontological argument is sound if maximal greatness is possibly instantiated; whoever accepts the possibility premise must accept the conclusion, and the possibility premise is more plausible than its denial. The argument shows the rationality of theistic belief; it does not compel assent from one who finds the possibility premise less plausible than its denial.5

Sources

  1. On my account the former is true but the latter flatly false, in view of the existence of possible but unactual states of affairs. What the actualist holds is not that whatever there is, is actual — after all, there are merely possible states of affairs — but that whatever there is exists. Actualism should really be called 'existentialism'. By now, however, it is too late; 'actualism' is already entrenched.
    Alvin Plantinga, p. 92
  2. My view may therefore be described as a sort of modal realism. Secondly, I took a possible world to be a way things could have been or a state of affairs — something like, for example, Socrates' being shorter than Plato or Quine's being a distinguished philosopher.
    Alvin Plantinga, pp. 88–89
  3. Hence there really does exist a being who is omniscient, omnipotent, and morally perfect and who exists and has these properties in every possible world. Accordingly these premises, (25), (27), and (28), entail that God, so thought of, exists. Indeed, if we regard (27) and (28) as consequences of a definition - a definition of maximal greatness - then the only premise of the argument is (25).
    The Analytic Theist, pp. 114–115
  4. An object has a property accidentally if and only if it has it and has a counterpart that lacks it; an object has a property essentially if and only if it and all its counterparts have it. Possible worlds, therefore, are maximal objects. The actual world is the maximal object of which we are parts; other maximal objects and their parts are possibilia.
    Essays in the Metaphysics of Modality, p. 206
  5. Neither are they nearby. They are not at any spatial distance whatever from here. They are not far in the past or future, nor for that matter near; they are not at any temporal distance at all from now. They are isolated: there are no spatiotemporal relations at all between things that belong to different worlds.
    Essays in the Metaphysics of Modality, p. 205

Where the argument is made

  • The Nature of Necessity (1974) — the systematic statement, with Chapters I–IV on de re / de dicto, haecceity, and possible worlds
  • Chapter VIII on transworld identity
  • Chapter X on the modal ontological argument
  • Essays in the Metaphysics of Modality (2003, ed. Davidson) collects the journal papers — 'De Re et De Dicto' (Nous 1969), 'World and Essence' (1970), 'Actualism and Possible Worlds' (1976), 'The Boethian Compromise' (1978), 'De Essentia' (1979), 'On Existentialism' (1983), 'Reply to John L. Pollock' (1985), 'Two Concepts of Modality: Modal Realism and Modal Reductionism' (1987)
  • the 'Self-Profile' (1985, Profiles 5) Sections 3C–G on the ontological argument, de dicto, de re, names, and possible worlds

How the argument was received

The actualist account is one of two dominant positions in modal metaphysics; the modal- realist rival is David Lewis's, on which possible worlds are concrete and we are the actual-world residents only indexically. Among actualists, the existentialism / anti- existentialism debate (with Robert Adams on the role of haecceities and propositions about non-existents) continues. The modal ontological argument has the standing of a formally sound argument whose possibility premise is contested; many philosophers find it begs the question, and many of those find that judgment less obvious once they have worked through it. The Essays in the Metaphysics of Modality (2003) collection gathers the technical record. The position has mostly held: actualism, transworld identity, and the broad shape of the apparatus survived Lewis's challenge and have been the working assumption of much subsequent modal metaphysics.