Philosophy
The Metaphysics of Possibility and the Transworld Identity Problem
Quick fact
David Lewis famously rejected the idea that the same individual exists in multiple possible worlds, instead proposing 'counterparts'—distinct but similar individuals in other worlds—to avoid the transworld identity problem.
Why this is interesting
You might think that you could have been a teacher instead of a programmer—but in what sense is that 'you' in that other scenario? What makes it the same person?
Read the full explanation
Understanding The Metaphysics of Possibility and the Transworld Identity Problem
To make sense of statements like 'I could have been a doctor,' philosophers use possible worlds—complete ways things could have been. The transworld identity problem asks what it means for an individual to exist in more than one of these worlds. For example, consider a scenario where you became a doctor. Is that person in that world literally you, or a distinct individual who is very similar? This puzzle arises because different worlds may involve different properties: in one you are tall, in another short; in one you have siblings, in another you are an only child. If identity requires having the exact same properties, then nothing could be identical across worlds. Philosophers have proposed essential properties—traits you must have in every world—to ground transworld identity, but it's hard to find such properties that are both non-trivial and sufficient for identity.
A deeper explanation
The transworld identity problem challenges the idea that individuals can exist in multiple possible worlds. The core issue is that if worlds differ in what is true of an individual, then the individual cannot have all the same properties across worlds, yet identity requires that. This is known as the indiscernibility of identicals. Two main responses emerge. Transworld identity persists: an individual is literally present in multiple worlds, and there must be a criterion for identifying that individual, often based on essential properties (e.g., origin, genetic makeup). However, this view faces the 'problem of accidental properties': if an individual can have different accidental properties in different worlds, how do we pick out the same individual? In contrast, counterpart theory, proposed by David Lewis, denies transworld identity. According to Lewis, each individual exists only in its own world; in another world, there is a counterpart—an individual similar in important respects, but not identical. This avoids the problem of identity across worlds but changes our analysis of modal statements: 'I could have been a doctor' means that my counterpart in some world is a doctor, not that I myself have that property in another world. This comparison shows how our theory of possibility is shaped by our commitments about identity and essence.