130 J.J. Haldane
cause of change could certainly not be dependent on any externalfactor,
nevertheless it might undergo modifications deriving from some internalsource.
There are, however, at least two (related) reasons for rejecting this. First, it
involves conceiving of the agent as composed of parts and this is at odds with
the idea of divine simplicity. Second, any ‘internal movement’ would give rise
to the sort of questioning that leads to the conclusion that there must be an
uncaused cause of change. Let me expand these points (and the relation
between them) starting with the second.
In presenting the prima via Aquinas writes that anything undergoing change
is being changed by something else (omne quod movetur ab alio movetur). We
will not really understand this claim and appreciate its force if we think solely
in terms of one object mechanically interacting with another – a polisher
shining a shoe, for example. Certainly this is a case of change deriving from
change; but to see the scope of Aquinas’s principle we have to recall his
analysis of change in terms of the transition from potentiality to actuality.
Prior to being polished, the surface of the shoe is dull but it has the possib-
ility of becoming shiny. In Aristotelian-cum-Thomistic vocabulary it is
actually dull but potentiallyshiny; or even more ‘scholastically’ expressed, it
isin act with respect to dullness and in potency with regard to shininess. This
situation will persist unless some factor operates to change it. Once that
factor comes into play the surface of the shoe is ‘moved’ from potency to act
with respect to shininess; or more familiarly, it becomes shiny. This ‘becom-
ing’ or realization requires an agent, and that agent cannot itself be wholly
potential, it must be active (or, equivalently, ‘in act’). So wherever there is a
transformation or a transition from one state to another some explanation is
called for of what effects this, and once that questioning begins it can only be
halted by coming to rest in an unchanging cause of change. The activity of
this primary agent cannot then be supposed to derive from either an external
or an internal source, for that would be to assume that it is not after all the
ultimate origin of change.
This reflection brings out part of what is meant by saying that God is
perfect. There is no scope for improvement in God or for any kind of devel-
opment, since this would be a change involving a transition from potentiality
to actuality in respect of some feature, and any such transition would then
require some prior actuality to initiate it. Young baby John grows through
taking in nutrients. The process of growth depends upon input from his
environment and upon internal physiological activity. These factors explain
the changes in John, but neither is itself wholly self-explanatory. Nothing will
constitute an ultimate explanation of change if it is itself subject to change
either from without or from within.
Reason brings us to a first cause of change and leads us to see that as such
it must be perfect and impassible – both in the literal and general sense of not