Transworld Identity

Definition and Background

Transworld identity is a technical term in contemporary modal metaphysics naming the putative identity of one and the same individual across different possible worlds. To say that there is transworld identity is to say that some object or person—this table, Aristotle, you—could have existed with different properties in another possible world, while still being numerically the same individual.

The term arises from efforts to formalize talk of possibility and necessity using possible-world semantics. When philosophers say, for example, “Socrates might have been a carpenter,” they seem to be talking about one and the same Socrates under different possible circumstances. The problem of transworld identity is whether such cross-world “sameness” should be understood as literal identity or instead analyzed in some other way.

The idea becomes particularly salient in the 20th century with work by Saul Kripke, Alvin Plantinga, and David Lewis, who develop different frameworks for interpreting de re modal claims—claims about what particular individuals could, must, or could not have been like.

Major Theoretical Approaches

1. Genuine Transworld Identity

Many philosophers adopt what is often called genuine or strict transworld identity. On this view, the same concrete individual can exist in more than one possible world (at least in the sense relevant to modal discourse), even if worlds themselves are treated as abstract representations rather than concrete realms.

• Kripkean frameworks:
  Although Kripke does not give a canonical definition of “transworld identity,” his theory of rigid designation suggests that proper names and some definite descriptions pick out the same individual in all possible worlds in which that individual exists. The same bearer appears under different property assignments across worlds. This presupposes a robust notion of cross-world identity.

• Plantinga’s essentialist approach:
  Alvin Plantinga explicitly defends transworld identity within a broadly actualist, possible-worlds framework. Individuals are said to exist in the actual world but are representable in other worlds as the same individuals with different accidental properties. For instance, Socrates could have been taller, but could not have failed to be a person. Here, transworld identity underpins the distinction between essential and accidental properties.

Under such views, questions arise about criteria of identity across worlds: what makes an individual in one world identical to an individual in another? Answers often appeal to some essential core—origin, kind, or haecceity (“thisness”)—that is preserved across worlds.

2. Lewisian Counterpart Theory

In contrast, David Lewis famously rejects strict transworld identity. In his modal realism, each possible world is a concrete, spatiotemporally isolated universe. Lewis argues that no concrete individual can exist in more than one such world. Thus, there is no literal transworld identity.

Instead, Lewis introduces counterpart theory:

• For any individual in one world, there are counterparts of that individual in other worlds.
• Counterparts are distinct individuals that resemble the original in contextually relevant respects (e.g., origin, psychological traits, physical structure).
• A statement like “Humphrey could have won the election” is analyzed as: Humphrey has a counterpart in some possible world who wins the election.

On this interpretation:

• Cross-world talk is paraphrased: what appears to be identity is really a counterpart relation—a relation of similarity and relevance, not of strict numerical identity.
• Transworld identity, strictly so called, does not exist. The term becomes a misnomer in Lewis’s system, which reconstrues the phenomena that motivated it.

3. Haecceitism and Anti-Haecceitism

Another fault line concerns haecceitism—the view that individuals have primitive, non-qualitative “thisnesses” that can vary across worlds independently of their qualitative properties.

• Haecceitists may ground transworld identity in these primitive thisnesses: the same haecceity appears in multiple worlds.
• Anti-haecceitists claim that all facts about possible worlds supervene on qualitative facts; there are no further identity facts “over and above” qualitative configurations.

Haecceitists typically find it natural to endorse genuine transworld identity; anti-haecceitists often find counterpart theory or purely qualitative criteria of cross-world comparison more attractive.

Philosophical Issues and Debates

1. Essentialism and Modal Properties

Transworld identity plays a central role in essentialist debates:

• If an individual is literally the same across worlds, one can compare its properties across those worlds to classify them as essential (present in all worlds where it exists) or accidental (present in some but not all).
• Proponents argue that this captures commonsense modal judgments (e.g., that an individual could have had different careers but not different biological parents).
• Critics contend that positing such cross-world identity facts is metaphysically heavy and may lack independent grounding.

In counterpart theory, by contrast, essentialist claims are reinterpreted as constraints on which counterpart relations are admissible, rather than as facts about a single thing persisting through different worlds.

2. De Re vs. De Dicto Modality

Transworld identity is often invoked to explain de re modality—modality about particular things:

• De re: “Aristotle might have been a fisherman.”
• De dicto: “It is possible that someone is a fisherman.”

Genuine transworld identity frameworks treat de re claims as directly about one individual across worlds. Counterpart theory interprets them as about that individual and its counterparts. Disagreement arises over which treatment better captures intuitive readings of natural language and the logic of modal statements.

3. Personal Identity and Moral Questions

Philosophers also explore transworld identity in relation to personal identity and moral responsibility:

• Could you have been a radically different person, with different memories, values, or physical form, yet still be you?
• If genuine transworld identity holds, one might say that “you” in another world bear moral properties (e.g., you could have committed a crime) that are ascribed to the same person.
• On counterpart views, these are properties of your counterparts, not literally of you, raising questions about how to interpret attributions of responsibility or regret like “I could have done otherwise.”

4. Semantics vs. Metaphysics

An ongoing issue is whether transworld identity is primarily a semantic tool or a deep metaphysical commitment:

• Some treat possible worlds as merely representational devices. For them, transworld identity may be a way of organizing discourse about possibilities without implying robust existence of worlds or individuals within them.
• Others, particularly realists about possible worlds, see the debate as a substantive question about what there is: are there literally the same individuals in different worlds, or only distinct but related individuals?

Debate continues over whether the advantages of genuine transworld identity (its intuitive fit with ordinary modal talk and with essentialist theorizing) outweigh the simplicity and ontological clarity sometimes claimed for rival accounts like counterpart theory.

In contemporary metaphysics, transworld identity thus names both a problem—how to make sense of cross-world sameness—and a contested solution, with different theorists either embracing, reinterpreting, or rejecting it in favor of alternative ways of understanding modal discourse.