In the following post, I offer a couple of comments of clarification on behalf of one of Aquinas's "Five Ways," his arguments for the existence of God that he offers in question 2 of the Summa Theologica. Specifically, I want to focus my attention on the argument from motion, i.e., the First Way. I hope to offer a more extensive defense sometime in the future. But, for now, my goal is comparatively modest. I flesh out the first key premise of Aquinas's argument: that "whatever is changing is being changed by something else" and I offer a response to three objections one might raise against it.
Aquinas begins the First Way with the hardly controversial
assumption that
It is certain, and evident to our senses, that in
the world some things are in motion.
He follows this observation with one that is not so
obvious.
Now whatever is in motion is put in motion by
another ...
Aquinas realizes, of course, that he will have to
provide support for this second claim, and he continues by clarifying what he
takes motion to be, for the purposes of his argument.
... for nothing can be in motion except it is in
potentiality to that towards which it is in motion; whereas a thing moves
inasmuch as it is in act. For motion is nothing else than the reduction of
something from potentiality to actuality.
Motion, for Aquinas, means in this context something
significantly broader than local movement or motion from one location to
another. Here, Kretzmann provides a useful short summary of what Aquinas has in
mind, based on his discussion of a number of parallel arguments from motion
from Aquinas's Summa Contra Gentiles:
As for Aquinas's identifying 'motion' as the basis
of these arguments, his use of the Latin word motus in them parallels
Aristotle's broad, generic use of the Greek word kinesis to mean either change
or location (local motion) or qualitative change (alteration) or quantitative
change (increase or decrease).[1]
The principal example that Aquinas draws on in the
First Way is based on motion in the sense of alteration -- in this case, fire,
which is actually hot, moving wood, which is potentially hot, to becoming
actually hot. But one should keep all three concepts in mind, understanding
motion in the rather broad sense that Kretzmann indicates, in order not to
weigh down Aquinas's argument from straw man criticisms.
The
idea that ties together the three senses in which Aquinas thinks of something
as in motion is the claim he makes that "motion is nothing else than the reduction of something from potentiality to actuality"
(emphasis mine). What Aquinas has in mind by "motion," then, is
something's being in a state of transition between potentiality and actuality,
taken in the sense of correlative concepts of two distinct modes of being from Aristotle's metaphysics. One
can elucidate Aristotle's concepts adequately, for present purposes, by
illustrating potentiality and actuality in light of Aquinas's claim that
But nothing can be reduced from potentiality to
actuality, except by something in a state of actuality. Thus that which is
actually hot, as fire, makes wood, which is potentially hot, to be actually
hot, and thereby moves and changes it.
A chair, for example, is either actually orange or
not actually orange. Being something that can
be orange, moreover, it is thereby potentially orange, though it is not, say,
potentially liquid, since nothing that is a chair can become liquid without
thereby ceasing to be a chair. Thus, if the chair is potentially orange but not
actually orange, it can be moved by something that is actually orange to
becoming itself actually orange. What cannot take place, however, is that a
chair that is only potentially orange moves itself to becoming actually orange.
For,
if a chair could move itself from potentially orange to becoming actually
orange, the chair, then, is either already actually orange or it is both
actually orange and potentially orange. But, Aquinas rightly notes,
... it is not possible that the same thing should be
at once in actuality and potentiality in the same respect, but only in
different respects.
And this is simply an application of the law of
non-contradiction. Thus, he continues,
It is therefore impossible that in the same respect
and in the same way a thing should be both mover and moved, i.e., that it
should move itself,
and this is not possible. Here, his reasoning seems
to be as follows: if a thing could move itself from being potentially F to
becoming actually F, it would already have something in virtue of which it was
actually F by which it could move itself from being potentially F to becoming
actually F. But, by stipulation, it is not actually F but only potentially F. So,
it is not the case that it moves itself from being potentially F to becoming
actually F.
One
might try to object to Aquinas's argument so far by insisting that it is not
the case that everything is either only potentially F or actually F. For some
things are only partly F and they are therefore partly not-F. In response, one
could reply on Aquinas's behalf by conceding that something can be partly F and
partly not-F, but also point out that if it is only partly F it is also not
actually F (nor is it actually not-F). And if it is not itself actually F, it
cannot move itself to become actually F. It therefore is made actually F if and
only if something that is itself actually F moves it from being potentially F
to becoming actually F. But if it isn't actually F nor is it actually not-F, what is it? The proper response to the question, I submit, is that it is F only in the sense that it is becoming-F. Something that is partly F and partly not-F is itself, necessarily, something G such that something actually F is moving it to become itself actually F such that, when it becomes actually F, it ceases to be a G. The change in question, in other words, is what Aristotle would call substantial change, in which a new individual of a certain species comes into being.
Another
possible objection is advanced by William Rowe in his book-length study of the
cosmological argument. One may state his objection as this: say that F stands
for "dead." Now, if something is in the state of dying, and the
transition from not-dying to dying is a change, it seems that something that is
dying must be moved towards the state of dying by something that is actually
dead.[2] But
this is plainly false. In reply, one might argue that "dead" is not,
properly speaking, a state of actuality for Aquinas. So nothing could be
actually dead in the sense in which something could be actually hot. There is,
however, a sense in which something is moved from being alive to becoming dead in the
sense in which anything that is alive is essentially F, where F is the essential form in virtue of which the thing in question exists as a
particular F. Hence, anything in terms of an accidental form that is actually G such that G is incompatible with
F will move something that is actually F towards becoming actually G. And in
moving something from F to G, the thing is per
se moved to becoming actually G and is per
accidens moved to becoming "actually" dead, i.e., from F to
not-F. Once again, the kind of change in question is not qualitative change, in which a substance gains and/or loses an accidental quality, but substantial change, in which a new individual substance comes into being and the old one goes out of being.
Another
objection to Aquinas, also from Rowe, is that there are other counterexamples
in which something that is not-F can bring something else that is not-F (and is
only potentially F) to a state of being actually F. For example, one can
produce fire by rubbing two sticks together, neither of which are actually on
fire. The proper response to this last objection, I think, is as follows.
Though, indeed, there is not something that is itself actually F in virtue of
which some thing brings it about that another thing is moved from being
actually not-F to becoming actually F, there is still a sense in which the
(first) thing has a power in virtue of which it can bring about F. One might
express this by claiming that the thing in question is actually E such that
anything that is actually E has the power to actualize the potency for F in
something that has that potency. Thus, F is in some sense "contained"
in E.
Rowe
anticipates this move, however, and claims that it would undermine the
principle Aquinas wants to establish, because, now, it is not clear why
something could not move itself to become actually F. My response is that if
something has the power to produce E such that anything that is E produces, in
any suitably disposed patients, an effect F, it will follow either that E
produces F in E itself, in which case the thing will be itself actually F (full
stop), and thus can reduce other things from potency to act F, or that the
thing requires another agent D such that D can act upon E in such a way that E
naturally produces F. In this last case, however, it is not in virtue of its
being actually E that the agent moves something from potentially F to becoming
actually F, but rather D's acting in conjunction with E.
However,
if the agent is actively E such that anything
that is actively E produces F in suitably disposed patients, then if E is also
a suitably disposed patient with respect to F, E will (without further
obstructions in place) naturally or necessarily produce F in itself. Hence, E
must have the power for producing F in some other sense besides being actually
F, or all E's will be such that they necessarily become F's. But, clearly, they
do not become all F's. Hence, it must possess the power for producing F in some
other sense. But it cannot activate this power on its own, i.e., move itself to
produce F, or else it would just naturally or necessarily produce F and hence
would itself be F. So, therefore, it is moved to produce F by another agent.
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