ARGUMENTS FOR MONOTHEISM 189The issue of whether there can be self-moving or self-changing things was
hotly disputed in the medieval period. Scotus, for example, offered
powerful arguments against Aquinas in this regard. But there is to be a
replacement for premise 2 that restricts its basic idea to cases of which it is
true, and we will follow this strategy.
Self-change
The First Way, if successful, would establish the existence of a cause of
change that did not change.^17 Premise 2 – the denial of self-motion – is
apparently problematic. There seem to be cases of self-movement – one’s
walking to the store, for example – and of self-caused change that does not
involve movement from place to place – deliberately stopping thinking
about one thing in order to reflect on another. But there are lots of changes
in which something is caused to change by something else, and the best
strategy for defending Aquinas here seems to be this: argue that, with
respect to the sorts of qualities with which the arguments are concerned,
self-change is not an option. Something depending for existence on itself –
a physical object or a human mind, for example, having some feature by
virtue of which it existed independently of anything else – is not a
promising notion. Perhaps the simplest defense of Aquinas here is to argue
as follows: we know that every physical thing, and that every human
mind,^18 in fact depends for its existence on the existence of other things.
But being dependent is an essential property of everything that has it; X is
dependent for existence on something else entails Necessarily, X is
dependent for existence on something else. Further, Nothing can cause its
own existence is a necessary truth; in order for something to cause its own
existence, it must do so at some time T. Then, at some time T, something
that causes its own existence must both not exist in order to be caused and
exist in order to do the causing – a feat that is logically impossible to
perform.
Even if one provides this line of reasoning on Aquinas’s behalf, however
- even if one grants that a version of, or replacement for, premise 2 that is
restricted to some such quality as non-reciprocally causing change will
serve his purposes even if some instances of self-change are possible – his
case is not made. Consider this pair of claims:
T1: For any time T1, everything that exists at T1 depends for its existence
on something else that exists at T1.
T2: For any time T2, everything that exists at T2 was caused to come into
existence at T2 by something that existed at T1.