20 Free Will, Moral Responsibility, Determinism
a future instant of the entire universe by Q, where P & L do not imply Q.
So even if determinism is true and this can be expressed as metaphysically
necessary conditional truth, determinism itself won’t be metaphysically
necessary.
Physical possibility is thus more restrictive than metaphysical possibility: the
set of all physically possible worlds is a proper subset of the set of all metaphysi-
cally possible worlds. Some worlds that are not physically possible are meta-
physically possible. For instance, while it is not physically possible for
something to travel faster than the speed of light, it is (arguably) metaphysically
possible. Even if a space ship traveling faster than the speed of light is meta-
physically possible, it is not physically possible because it is inconsistent with
the actual laws of nature. And even if Napoleon’s winning the Battle of Water-
loo is metaphysically possible, it is not (now, in 2015) physically possible
because it is inconsistent with the actual past.
We are now in a good position to make another relevant distinction. Note,
first, that traveling faster than the speed of light is metaphysically impossible
holding fixed just the laws of nature, irrespective of holding fixed the past. We
might therefore say that this state of affairs is also nomically impossible—that is,
impossible with respect to the laws alone. However, some states of affairs will
be physically impossible but nomically possible. Imagine that currently Juan is
in Tucson, Arizona, and we ask whether it is nomically possible that he currently
be in Buenos Aires instead. It is. The laws of nature, irrespective of the past, are
compatible with his being in Buenos Aires right now. Nevertheless, it is physic-
ally impossible that Juan be in Buenos Aires right now. That is, it’s metaphysi-
cally impossible holding fixed the laws of nature and the facts of Juan’s recent
past that he currently be in Buenos Aires instead.^19 The lesson to learn here is
that physical possibility is a function of what the laws of nature permit given
certain fixed details about how physical reality has been arranged. And we can
see that while in one sense it is not possible that three seconds from now Juan
will be in Buenos Aires, there is another sense in which it is. It is nomically pos-
sible. Consistent with the laws of nature, he could have arrived there three
seconds from now had he bought his ticket on time.
Consider now the following issue, which often arises in the discussion of
determinism as it relates to action. Suppose that Dana earlier this morning drove
her children to school, and assume that our world is deterministic. On our char-
acterization of determinism, it would then be metaphysically necessary that
holding fixed the past and the laws of nature that Dana drove her children to
school at this time. Are we now also forced to say that it is metaphysically
necessary that she did so? That it was not even metaphysically possible that her
husband Sam took the kids to school? How could this be a consequence of deter-
minism? We are not forced to say these things, and getting clear on why will
help clarify the relevant sense of physical impossibility at issue.
At this point it’s important to understand how certain modal inferences work.
Modality concerns different sorts of necessity, possibility, and impossibility, and
so we have in this section and the preceding one been discussing a set of modal