That something is moved (aliqua moveri), Aquinas claimed, was certain and apparent to the senses (ST 1a Q2, art. 3). That such motion could be used to demonstrate the existence of God, Aquinas also claimed, was the more obvious (manifestior) of the five ways he used to do so (Ibid). Yet which kinds of motion were included within the proof. Whether it was the thing in motion’s being moved or its being in motion which required a mover. Whether the mover’s causal activity was simultaneous or successive or even extended to the moved thing’s causality. These and other such claims became the subject of much dispute in the interpretation of Aquinas’s proof from motion, as did the question of its validity, and its relationship with modern science. It is to the assessment of such claims, therefore, that Daniel Shields turns in his new book about Aquinas’s proof from motion, Nature & Nature’s God.
Shields divides the work into two parts. The first and larger part consists of five chapters and examines the proof as such. In the first chapter, accordingly, Shields investigates the proof’s first premise, omne quod movetur ab alio movetur: the mover principle as it is better known. At issue is the meaning of ‘movetur’: should its first occurrence be translated as ‘is moved’ or ‘is in motion’? This is not scholastic obfuscation, however. Whether it is the being moved or the being in motion which requires a mover generates two philosophically quite different versions of the mover principle. On the former interpretation, only those things now being moved require a mover. On the latter, it is everything now in motion.
Shields opts for the latter interpretation (p. 23) and puts forward two main arguments in support of his contention. The first is textually based and draws upon the proof from motion in the Summa contra Gentiles where Aquinas had remarked, ‘that some things are in motion – for example, the sun – is evident from sense’ (ScG I. 13). What makes the sun’s motion apprehensible by the senses is not, Shields suggests, ‘that the sun is passively being moved … but … that the sun is in motion’ (pp. 22–23). So, according to Shields, the evidence of the senses attests to the sun being in motion; ‘movetur’ ought therefore to be translated as ‘is in motion’, and the mover principle extends to whatever is in motion.
Still one can take issue with this approach. Set aside the fact that the sun is not actually being moved round the earth and ask instead what would be apparent to the senses were such ‘motion’ to occur? Surely what one would sense was that the sun appeared constantly to be changing place. But if the sun or indeed any other celestial body was constantly changing place would it not have to be moved in order to do so? Motion, after all, is not an end in itself. Further and more fundamentally, it is not clear that sense knowledge alone can solve this problem. Aquinas had remarked that motion was a common sensible (In II De Anima, lect.13 # 386). As such it is, like any common sensible, something about which the senses can be in error (ST 1a Q85 art. 6). Thus when Aquinas says that motion is ‘certain and apparent to the senses’ (ST 1a Q2, art. 3) such certitude cannot derive from sense knowledge. It has to be a consequence of intellectual knowledge, in which case, however, the intellect is involved in the claims Aquinas is making about motion, and it is erroneous to suppose that sense knowledge alone can be used to adjudicate the meaning of ‘movetur’, much less how the mover principle should be interpreted.
Shields’ second argument proceeds from the natural motion of inanimate bodies. Aquinas, as is well known, had insisted that such bodies would spontaneously move towards their natural places unless impeded (In II Phys. lect. 1, # 144). Their per se mover would be their generator (Ibid.), but once generation was complete and natural motion had begun, what would be the cause of that motion? Shields holds that the being-moved interpretation of the mover principle fails to provide an explanation. Not only is the moving body no longer in contact with its generator, but also once enough time has passed the generator will no longer exist (p. 30). Shields, accordingly, proposes a revision of the mover principle. It is, he suggests, better expressed as, ‘everything in motion has an efficient cause of its motion, whether currently in contact with it and exerting force or not’ (p. 24). So understood, Shields can argue everything in motion has a mover but crucially that the mover need not be in contact with the moved thing for every moment of its motion. Such a mover determines the motion (p. 33). It is, in Shields’ words, the ‘responsible agent’ (Ibid.).
But responsible how? Shields cites QDP Q3 art. 11 and argues that the substantial form of the moved thing is the mover’s instrument (pp. 34–35). This is a surprising development, however. Typically when Aquinas speaks of instrumental causality he contrasts proper and instrumental activity, attributing the former to the instrument’s form and the latter to the power of the principal agent (ST 3a Q62 art. 1 ad 2). However, if the substantial form of the moved thing is the mover’s instrument, then the moved thing’s putatively proper activity will in fact be instrumental, in which case a moved thing’s putatively proper activity will in no sense be commensurate with its nature and as a result lead to no knowledge of that nature. In effect natural philosophy would become pointless.
Shields considers the proof’s second premise, the denial that an infinite regress of moved movers is possible, in chapter two. Whether Aquinas had intended this denial to pertain to a moved mover’s being moved or its being a mover is not clear. Shields, however, opts for being a mover and therefore renders the proof from motion a more restricted version of the second way (p. 47). One might question this, though. After all, if the first way is just a more restricted version of the second way, why would Aquinas have presented it as a distinct proof?
Be that as it may, Shields’ central claim in the chapter is that the series of movers/causes need not always be simultaneous (p. 48). Essentially Shields’ position is dependent upon two claims. First, that ‘in essentially ordered series, all caused causes fail to explain their effect … because they do not have within themselves the ability to produce the effect … they can only do so insofar as they are caused to cause it … caused causes have the status of instruments’ (p. 53). Second, that an instrument does not require constant contact with its principal mover (p. 34). Both claims are problematic, however. To the first, one might object that Shields needs to substantiate his position by arguing either that (i) per se causes cannot be essentially ordered, or (ii) that if they can, that they fail to explain their effect. To the second, one might reiterate the aforementioned points about instrumental causality. In addition, one could suggest that QDP Q3 art. 11 was making a general claim about moved movers, only some of which were instruments, and therefore might equally be read as supporting the being moved interpretation.
Chapters three to five examine the various qualifications and developments Shields proposes for the proof. Chapters six to eight discuss it in the light of contemporary science. Lack of space prohibits further discussion of the many interesting issues raised, but Shields has done a service to Thomism by reminding Thomists of the possibilities inherent in the proof from motion.
