Saturday, August 16, 2008

Can There Be a Self-Mover? Aquinas on Act and Potency

My family and I are now living in South Bend, Indiana. We survived the move well enough, though unfortunately some of our furniture didn't. (The movers wrecked the baby's crib, broke all four of our floor lamps, and lost a box containing three of wall pictures. Next time we go U-Haul.)

Anyway, my first blog post in a long time concerns Aquinas' distinction between act and potency. More specifically, I'm concerned with Aquinas' claim, central to the first two of his "Five Ways" of proving God's existence, that

(1) Whatever is moved is moved by something else.

If this is right, then there can be no self-movers. In the Summa Contra Gentiles Aquinas presents three arguments for (1). None of these strike me as particularly convincing, but I want to focus for a bit on the third argument, which appeals to the act/potency distinction. Basically, Aquinas argues as follows:

(2) Nothing can be both in act and in potency in the same respect at the same time.
(3) If there were a self-mover, then something would be both in act and in potency in the same respect at the same time.
(4) There cannot be a self-mover.
(1) Whatever is moved is moved by something else.

While I accept the validity of this argument, I am doubtful of its soundness because I find it very hard to come up with interpretations of 'act' and 'potency' that make both (2) and (3) true.

My first thought was that 'act' means actuality and 'potency' possibility, such that for something to be in potency with respect to property F is for it to be possibly F. On that reading, however, premise (2) is false. Since actuality implies possibility, it is perfectly possible - indeed necessary - that if X is actually F at time T then X is possibly F at time T.

My next thought was to make premise (2) true by defining 'potency' as possibly but not actually. The problem is that now (3) is false. To see this imagine a hypothetical situation in which a self-mover M can move either to the left or to the right. Initially, M is in 'potency' (as we have defined it) with respect to both options. That is, both options are possible for M, but neither is at yet actual for M. Now suppose that M moves itself to the left. In that case, M ceases to be in 'potency' with respect to either option and is in 'act' with respect with to going to the left. All this seems perfectly consistent, which falsifies (3). Why? Because the shift from being in 'potency' w.r.t. going to the left and being in 'act' w.r.t. going to the left is diachronic. M is never in 'act' and in 'potency' in the same respect at the same time. This indicates that it is possible for the antecedent of (3) to be true while its consequent is false, which is sufficient to show that (3) is not a necessary truth.

My final thought was to interpret 'act' and 'potency' in a causal sense of active and passive, respectively. This plausibly renders (2) true, though the proponent of self-motion might object that it begs the question. But my main worry again is that (3) is false. Why can't self-motion be understood in terms of acting at T1 so that M's future self is F at T2? Aquinas' argument against self-motion only works if a self-mover would have to be both in 'act' and in 'potency' in the same respect at the same time. But I just don't see why the 'act' / 'potency' relation can't be understood diachronically rather than synchronically. Indeed, shouldn't the very act of self-motion shift the temporal index forward? If so, then (3) is false and with it falls one of Aquinas' arguments in favor of (1).


At 8/16/2008 6:55 PM, Blogger Brandon said...

I'm a little unclear about your third argument (interpreting it as active/passive). If we then understand self-motion as "acting at T1 so that M's future self is F at T2," what are we taking as the thing that is being acted upon (and thus passive)?

At 8/17/2008 1:18 PM, Blogger Jessica said...

Your second account of what potency means is the correct one. Potency as such is potency with the privation of the corresponding act.

Your argument, however, considers "going to the left" (a clear example of a motion) as an act without qualification, whereas St. Thomas explicitly denies that motion is an act without qualification. If you say motion is an act, you're simply denying St. Thomas's definition of motion altogether. But so long as you're assuming that St. Thomas is wrong about motion, your argument begs the question.

At 8/17/2008 4:35 PM, Blogger A thomist said...

I agree with Jessica. You give the right account of potency the second time, but your account of motion is not St. Thomas's or Aristotle's. You see motion (moving to the left) as an immediate transition from potency to act, such that there is no transition or "eduction" from what is simply potential to what is simply actual. But the point of the description that St. Thomas gives of motion as "the eduction from potency to act" is that motion is between potency and act. It is neither act as such nor potency as such but the act of the potential with privation.

Briefly, your division that either potency has act or doesn't (your first and second accounts) leaps over exactly what Aristotle and St. Thomas mean by motion.

St. Thomas is giving only a single argument that Aristotle gives in his Physics, and he does not use the one that Aristotle considers most scientific and demonstrative. Aristotle argues that the motion of a body is caused by its parts, and parts must be composed by another.

At 8/18/2008 2:50 PM, Blogger Alan Rhoda said...

Fair question, Brandon. Active/passive may not be the right terms for what I had in mind. What I was think of was more like cause/effect. Effects are often, and perhaps always, subsequent to their causes.

At 8/18/2008 3:22 PM, Blogger Alan Rhoda said...

Thanks Jessica and A Thomist, for clearly singling out my second idea as the correct one.

Your joint objection that motion (as Aquinas conceives it) isn't a pointlike 'act' or 'event' but rather more like a continuous 'process' is an interesting one. It is plausible to think that the latter reduces to the former. Processes, that is, seem to be sequences of events. If that's right, then my objection to Aquinas stands, though the requisite level of analysis may need to be much more fine-grained than my example suggests.

Take any continuous process and freeze it at an arbitrary instant, call it T1. At T1, some determinate state, call it S1, is realized. Hence S1 is 'in act' with respect to a property, call it F1, that describes the way it is. But S1 is a dynamic state, the state is about to change, so it is T1 also 'in potency' with respect to becoming a different state, S2, characterized by a different property, F2, at a latter time, T2.

In virtue of the fact that the temporal index changes with the motion, I think premise (3) still comes out false. Even if there is a self-mover, it will never be 'in act' and 'in potency' in the same respect at the same time.

At 8/18/2008 3:31 PM, Blogger Alan Rhoda said...

A Thomist,

The problem I have with Aquinas' argument that the motion of a body depends on motion of its parts (and hence is not properly self-motion) is that it is not at all obvious that there cannot be a simple substance (one that has no proper parts) that is a self-mover. (It is probably better here to speak of 'change' rather than motion.) Cartesian souls, angels, or God, for example, are partless, yet have been thought (not by Aquinas but by others) to be self-movers.

At 8/18/2008 3:44 PM, Blogger A thomist said...

Good to hear from you. I think this is the third time we've gone around on this question. I think we're speaking past each other. Let me focus the question. Consider "Going from left to right":

Aquinas: "That's the eduction from potency to act"

Rhoda: "That's an act".

This means you either

1.) Assume Aquinas is wrong about motion, or

2.) You are arguing that to go from A to B is the same as to be at B.

At 8/18/2008 3:50 PM, Blogger A thomist said...


Oh, that's the problem? Great!

In this sense of "self motion" St. Thomas insists that everything living, an not just subsistent forms are self movers. See ST I, Q 18. art. 3.

At 11/09/2008 3:28 PM, Blogger Clayton said...

Maybe this thread is moribund, but if I'm following the last few comments, this might be useful.

It's important to distinguish between the transitive and intransitive forms of 'move'. The natural reading of (1) is 'move' is the transitive reading. ('She moved the mug from the table' is transitive 'He moves poorly because of his size' is intransitive.) On this reading, something moving to the left is not a case of motion in the relevant sense whereas someone's being moved to the left _is_ the relevant sense. (Just think about it, (1) would be nonsense if 'moved' was read intransitively.)

Of course, as Kenny noted, there are problems with the argument if we read 'move' in (1) as the transitive form because then we have to avoid equivocating by reading 'move' as its intransitive form in the other premises of Aquinas' argument from motion.


Post a Comment

Links to this post:

Create a Link

<< Home