28 Free Will, Moral Responsibility, Determinism
18 Here is a note for advanced students familiar with some related topics in the meta-
physics of modality: Some philosophers worry that defining determinism as a relation
between propositions is misleading. Propositions are abstract entities. But determin-
ism is about the world; it is about concrete reality. It would be better to have a defini-
tion that expresses determinism about the world’s possibilities. Consider, then, this
definition, that is a rough paraphrase of the one David Lewis proposed (Lewis, 1973):
W1 is a deterministic world just in case there exists no other world, W2, that
shares with W1 at a moment in time, t, W1’s entire history and the laws of nature
so that until t, W2 is a duplicate of W1, and yet W2 diverges from W1 at some
later time, t + n, such that W1 and W2 are no longer duplicates.
Many will protest that talk of possible worlds is no better than talk of propositions.
Indeed. But Lewis’s proposed modal semantics can be exploited to articulate our
familiar modal judgments without buying into the existence of possible worlds. It can
be used as a useful heuristic that simply translates, for a deterministic world, d, as
this:
It is not metaphysically possible for this world, d, to have more than one future,
given its actual past and its actual laws of nature.
We will work with the informal formulation, D, offered in the text above. But we
have no allegiance to it, and although this Lewis- inspired formulation is not often
used, nearly every controversy we shall discuss that involves determinism could be
recast in terms of this definition.
19 A warning here: Sometimes, to capture this later sense of physical impossibility, phi-
losophers will, contrary to what we write here, say that it is nomologically impossible
in a case like this for Juan to be in Buenos Aires in three seconds. But what they mean
is that it is nomologically impossible, given Juan’s present location and recent history.
And so their full meaning is not just about what the laws of nature do and do not
permit but what the laws of nature do and do not permit given other facts about the
arrangement of physical reality. To keep these matters clear, we advise a more cau-
tious and regimented way of talking about these issues.