Sunday 18th September, 2016
A possible world is something like a worldbook – something that describes to the very
last details everything that is true of the world. Something is necessarily true if it is true in
every world. Something is possibly true if it is true in some world.
Epistemic vs. Metaphysical Possibility Tom Hanks might be an Alien Tom Hanks is
human, but he might have been an alien. Having a debate about the actual world. ‘For all
I know, Tom Hanks is an alien.’ Not discussion about how things actually are.
British people use ‘might’ more than ‘could.’
Water = H2O. Mohammad Ali = Cassius Clay. Kripke thinks that these are necessary
truths. But you could truly say ‘water might not be H2O’ even if it is true that water is
What is the subject matter of ‘possibility’? Framework of possible worlds doesn’t really
help this question. For, ‘what are possible worlds?’
Modal Realism: Not only are there things that are possible worlds, they are concrete
entities (just as our physical universe is a concrete entity) and exist in the same manner as
Main Aspects of Lewis:
(1) Possible worlds exist. (2) Same kind as actual world. (3) Causally isolated. (4)
Indexical analysis of actually. (5) Counterpart theory for de re modality.
There’s no way to causally interact with them, so why should we say that they exist?
Lewis’s argument. (1) Possible worlds exist. (2) Every alternative to taking possible
worlds as concrete entities is fatally ﬂawed. (3) Therefore, modal realism is true: the actual
world is just one of a plethora of concrete possible worlds.
‘Actually’ as an indexical
Indexicals: I, here, now
Represent self-locating information.
Talk about the actual universe restricts attention to this universe (that we occupy).
‘Actually’ means ‘in the possible world where I am located’.
Stalnaker’s Objection: Possible worlds are ways for a world to be. They are therefore
something abstract like properties, rather than concrete individuals.
The knowledge objection: If possible worlds are spatiotemporally and causally isolated
from the actual world, how could we know anything about them?
The objection from morality: If modal realism is true, it is impossible to make any
diﬀerence to the amount of suﬀering and joy that exists in reality. We might as well give up
caring about anything.
Ersatz Possible World: An abstract stand in for a concrete possible world that represents
in some way everything that would be true in a concrete possible world.
Linguistic Ersatzism: Take the ‘worldbook’ idea more seriously. Possible worlds are
‘maximal’ sets of consistence sentences or propositions.