202 J.J.C. Smart
the restricted domain of classical mechanics, both the actual world and possible
worlds would be lines in phase space. If I were a possible world enthusiast,
I would (despite Lewis’s reservations) prefer to be an ersatzist. Plantinga has
his own form of ersatz world, something which he calls a ‘book’ – a set of
propositions such that for every proposition either it or its negation occurs.
Suppose that we say that either ‘God exists’ or ‘It is not the case that
God exists’ occurs in the book. Well, he doesn’t really exist if he exists only
in the ersatz world (say, the book or a mathematical model). Surely in that
case only the representation of God belongs to the ersatz world: ‘z exists in
ersatz world w’ is a systematically misleading expression (as Gilbert Ryle
might have put it).
Most theists would prefer ersatzism to Lewis’s realism about possible
worlds. Most theists believe that theism has something to do with morality,
so that God would have an interest in whether I boil my grandmother in oil
or refrain from so doing. According to modal realism God should not be
interested. If God creates all possible worlds (as I suppose a theistic modal
realist would have to say), it does not matter whether I boil my grandmother
in oil or whether I don’t. In the set of real possible worlds there would be a
world in which I do and another in which I don’t. A sufficiently hard-nosed
Calvinist might respond by removing benevolence from his list of God’s
excellences. After all the Calvinist view is that salvation is by grace not works,
and people are predestined to heaven or hell, which does not sound like
benevolence. Lewis is not a theist, but he avoids this worry about benevolence
because he has a parochial interest in the actual world, just as an extreme
nationalist might have no moral concern about what goes on in other coun-
tries. Lewis rejects H. Sidgwick’s wish to look on morals ‘from the point of
view of the universe’ (for the modal realist the universe would contain all the
real possible worlds). Still this is a digression in being an argumentum ad
hominemto a certain sort of theist. We can discuss Plantinga’s argument,
which refers to God’s excellences, whether or not we use moral predicates in
defining excellence.
The favourite modal logic system for propositional modal logic is
C.I. Lewis’s system S5 and Plantinga’s is such a system in its propositional
fragment. I shall, first, state Plantinga’s argument with reference to his pos-
sible worlds semantics for quantified modal logic (i.e. logic which counten-
ances the words ‘possibly’ and ‘necessarily’withinthe scope of words such as
‘some’ and ‘all’). However, I shall go on later to state what I think is the main
issue more simply in terms of propositional modal logic. Plantinga introduces
the notion of a maximally great being and plausibly argues that maximal great-
ness implies maximal excellence. To be maximally great is also to exist in every
possible world. Then he invites us to agree that a maximally great being is
possible. Being possible it exists in at least one possible world, and being
maximally great at that possible world it exists at all possible worlds.