16 Free Will, Moral Responsibility, Determinism
1.4. Determinism
In its simplest form, determinism is just the thesis that at any time only one
future is physically possible (Mele, 2006b: 3; van Inwagen, 1983: 3).^9 Indeter-
minism is then the denial of this thesis—it is the claim that at some time more
than one future is physically possible. Determinism is an entirely general thesis.
It applies to any aspect of the natural order whatsoever. If tomorrow a few grains
of sand on a riverbed are moved by the river’s current exactly three inches
downstream, on the assumption that determinism is true this is the only physic-
ally possible trajectory for those grains. Similar remarks apply to any region of
space over any stretch of time for any object occupying that region. This
includes the length of each hair on your head, the amount of moisture currently
in your eyes as you read this sentence, and it also includes each action you
perform, and each state your body is in at any moment. If determinism is true,
then in one important sense, each action you perform is the only action that it is
physically possible to perform.
There is hidden complexity in the preceding formulation. The key to this
complexity lies both in the notion of physical possibility and in the ground for
only one future’s being physically possible. We’ll turn to these matters shortly,
but we’ll start here with a transparent detail of the formulation we have before
us. As set out, causal determinism is a thesis about any time—and so ranges over
all times. A different way to define causal determinism is to begin not with a
claim about all times, but with a statement about what is involved in an event’s
being determined.^10 So, consider this: An event is causally determined just in
case it is not physically possible that it not occur. And one event determines
another just in case, given the occurrence of the first, it is not physically possible
that the other not occur. Here, we do not have a definition of the thesis of deter-
minism; instead, we have definitions of what is involved in an event being deter-
mined, and what is involved in one event determining another. Determinism,
building on this definition, can then be defined as the thesis that every event is
determined by a prior event.^11
The second characterization illuminates a feature of causal determinism that
is important for the free will debate. Consider the following model of a deter-
ministic world, Wd, followed by a model of an indeterministic world, Wi. Let
“e” represent an event. Let “—” represent a causally deterministic (d) relation
between events. And let “.. .” represent an indeterministic (i) relation between
events. Now consider each world:
(Wd): e1—e2—e3—e4—e5—e6—e7—e8, and so on with only d relations(Wi): e1—e2... e3—e4—e5—e6—e7—e8, and so on with only d relationsNotice that in Wi there is only one pair of events, e2 to e3, that are indeterminis-
tically related; the rest are deterministically related, just as they are in Wd. Yet
Wi is a world in which determinism is false. Now for the reason this is