I am a physics student but very interested in some topic of philosophy (specially in analytic philosophy). A question which have been struggled me for some time is the relation between modal concepts such as necessity, possibility and … which their semantic are understood in terms of possible world interpretations. In a causal world, I don't know how possibility can be imagined. We have not a true counterfactual situation (a possible world) where it's past is the same as our actual world but the future will be different (I assume that the past of possible worlds are the same because we are thinking about same objects in the different possible worlds). Is causality consistent with modal concepts? For example, most ( may be all) philosophers say that "Alex is a writer" is a possible proposition for Alex and he may not have that property, but in a causal world, Alex necessarily has this property and it is fixed by events in the past and there is no room for other choices.

My question is as follows:

If we consider Alex in two different possible world, how can we argue that these two guys are the same person? I can imagine different possible worlds, but all objects are different in these worlds.

  • It sounds like you're suggesting that the version of Alex who is a writer is a "different person" to the actual version of Alex. If so, you may find this useful, but it's a very broad topic.
    – Kevin
    Commented Mar 30, 2022 at 21:05
  • @Kevin, thanks, I was thinking last night that causality and possibility are not contradict each other. There might be two different world with the different causes and sharing the same objects, do you agree that two possible world with the same past's causes have to have the same future? because all future actions are constraint by past's causes. So if Alex wanted to be a writer, one of the causes had to change before his decision .
    – Arian
    Commented Mar 31, 2022 at 9:08
  • That is only true if you assume a deterministic universe. Any amount of randomness, no matter how small, allows for universes with identical pasts to develop different futures.
    – Kevin
    Commented Mar 31, 2022 at 16:27
  • There is only one actual world and only one actual Alex. All the possible worlds and Alexes are just ideas about what the future might bring. Commented Aug 30, 2022 at 9:30

3 Answers 3


Clearly this is a complicated question and don't think there is widespread agreement about it in philosophy. My own take is that we should think of language that picks out entities in possible worlds as indexed de re to the relevant object in the actual world. We can contrast this with the theory that identity of entities and their possible counterparts is done de dicto by similarity under some weighted set of properties.

To understand the de dicto/de re distinction consider a simple example that does no involve possible worlds (at least not explicitly):

While, I'm organizing my bookshelf I notice that a book has been jammed in between the couch cushions. Being lazy, I ask my wife, "could you get me my copy of Reasons and Persons over there in the couch". She lovingly goes to get the book, but lo and behold when she pulls it out, it is not Reasons and Persons after all but rather The Girl with The Dragon Tattoo. She hands me the book. Now, if I was referring to the book based on the logical extension of the description (de dicto), "my copy of Reasons and Persons over there in the couch" then the book I was handed was not the book I meant because it lacked an essential property designated in the description of being a copy of Reasons and Persons. However, more likely, I was just using the description "my copy of Reasons and Persons in the couch" to indicate the actual book I wanted her to get which as it turned out was The Girl with the Dragon Tattoo. My description was de re, the correct reference was based on the nature of the actual thing being indicated not a logical extension based on a set of properties given in a description.

How is all this relevant to possible worlds and Alex? When I say something like, "Alex is a lawyer in the actual world, but it is possible for him to have been a philosopher if only he'd not been pressured into law by his parents." A modal logician would model the meaning of this statement with a non-actual possible world w in which Alex is a philosopher due to having received less pressure from his parents in the past. You are asking, how do we know that philosopher Alex in w is the same person as Alex in the actual world. If we understand "Alex" as an abbreviated description, something like a weighted list of Alex's Alex-making properties, then clearly the Alex of world w and the Alex in the actual world both meet a large number of the properties on the list.

One account of transworld identity says that we should generate a weighted list of world-independent Alex-making properties which pick out Alex in the actual world. Then looking at all the entities in world w, find the entity that is the most Alex-like. As a caveat we can say that if no entity is sufficiently Alex-like, then we say that Alex does not exist in w. This is roughly a de dicto account of transworld description. It treats a possible world as an extensional totality, i.e., as just like the actual world in its logical structure except for the fact that it lacks the property of actuality (some even say that the property of actuality is merely relative to a chosen world where we evaluate the truth of non-modal statements).

An alternative is to see our talk of possible worlds as intensional and our ability to refer to entities in possible worlds as de re. When I refer to Alex in world w, I know it is Alex because I intended to talk about a variant of actual Alex. I am not discovering the Alex of world w as if I'm visiting an alien planet, I'm talking about a way that the actual world might have been if things had gone differently. When it comes to modal reference there is no need to act like a scientist and go searching for the most Alex-like entity in the whole universe of world w. After all, world w isn't a physical entity, it is only a logical construct necessary to build a semantic model of modal language. I get to automatically import my ability to refer to Alex from the actual world into any possible world (although in some worlds there might be no reference entity available).

Let's consider an example where the de dicto and the de re account could have different implications. If I say, "It is possible that Alex could have been born 10 years earlier than he in fact was." Now, if a certain account of personal identity is true, on one might think that anybody born 10 years earlier would not be the same person as Alex. We could imagine a person just like Alex in terms of appearance, personality, etc., but (barring time travel) this person would have originated from a different sperm and egg, so he would be numerically a different person. However, on the de dicto account of transworld identity, the older clone of Alex might be identified as Alex in world w, because older "Alex" is the entity in w who is most similar to actual Alex. Crucially, the older "Alex" does not lack any of the essential features for being the kind of thing actual Alex is (i.e., being a person). Viewed extensionally/de dicto there is no way to tell that older "Alex" is not a possible version of actual Alex.

On the other hand the intesional/de re account would say that when our language reaches across to another world, the meaning of that language is fixed by the actual reference entity (in this case actual Alex). Thus, when we consider the older "Alex" we aren't just matching entities of w against a world-independent basket of properties, we are determining whether older "Alex" is a possible way that actual Alex could have been. In this case we might determine that older "Alex" is merely a time-shifted copy of actual Alex, but not a possible version of Alex himself.

  • 1
    According to Lewis' recombination principle and supported by Stalnaker the de re intensional account resulting non-logical metaphysical necessity could be challenged, since the genetic essence of your Alex could be recomboed in a very remote PW, and thus only the 10 years earlier copy of Alex exists and the designated Alex doesn't. Similarly water could be composed like alcohol and thus not H2O necessarily in such a PW... Commented Aug 28, 2022 at 19:00
  • @DoubleKnot I'm not familiar with the recombination account. If you feel like explaining it, I'd be interested to know more.
    – Avi C
    Commented Aug 29, 2022 at 2:23
  • See this paper's abstract or you can simply google search using the keyword. Basically he asserts everything can coexist with anything else, any vice versa. To its logical end, some PW might not even be spatiotemporal. Thus Kripke's famous a posteriori metaphysical necessity Water is necessarily H2O doesn't exist... Commented Aug 29, 2022 at 2:57

Indeterminism: The cheap answer is to refer to the existence of indeterminism in the atomar region, for example the decay of a radium atom. According to the Copenhagen interpretation quantum mechanics is complete. There is no hidden parameter which determines the different outcome concerning two radium atoms prepared in the same way.

Many-worlds: More challenging is to point to Everett‘s many-worlds interpretation: Each event splits the given world into many worlds with different outcomes of the given event. Before the event all these worlds are possible. Of course the Everett interpretation – like the Copenhagen interpretation - is not accepted by all physicists.

No-clone theorem: A person who wants to keep determinism can argue that it is not possible to clone Alex. One cannot implement two identical persons with the same history.

Added: Those pointers to contemporary physics show: One has to consider some caveats when combining determinism and the concept of possible-worlds in a philosophical context.

  • Thanks, I am mostly interested in philosophical point of view of the answer, what you have mentioned is about different interpretations of quantum mechanics. I think possible world's semantic of modal logic is not depended on the content of physical theories (for example, no-clone theorem in quantum mechanics).
    – Arian
    Commented Mar 30, 2022 at 19:04
  • Arian, I understand your intention. My intention was to point out that philosophical considerations should take ino account the state of the art in contemporary science. Hence I indicated some caveats if one wants to combine in a philosophical context determinism and the possible-world concept.
    – Jo Wehler
    Commented Mar 30, 2022 at 19:24

Kripkes "possible worlds" is a kind of mathematical fiction. Its rather like the notion of a "universal set" when you can see quite correctly that the universe is still where it was before and hasn't been smuggled inside a set.

A universal set has references to all the things in the universe. But in no sense is it the universe. Likewise, the possible worlds are in no way actual possible worlds. They are a technical tool to work with the formal semantics of modal logic.

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