The Motion-Body Problem

The Motion-Body Problem

“For I would fain know what substance exists that has not something in it, which manifestly baffles our understanding. Other spirits, who see and know the nature and inward constitution of things, how much must they exceed us in knowledge?” John Locke, Essay Concerning Human Understanding, Book 4, chapter 3

Mechanism succeeded scholasticism as a theory of motion in the seventeenth century, to be displaced by Newton’s gravitational theory in the eighteenth. By postulating a force that acts at a distance Newton abandoned the restriction to contact causation, thus departing from commonsense physics. This theory was widely regarded as ”occult” (including by Newton himself) and as a regrettable recrudescence of scholasticism. Newton’s equations described the laws of motion but did not identify its cause, about which he declared himself ignorant. Mechanism had promised to render the world intelligible, but Newton’s mysterious force gave up on that ideal, resting content to offer mathematical predictions. So matters remain to this day (despite Einstein’s innovations). This is a familiar story, which I will not re-tell[1]; but it is worth dwelling on how strange Newton’s gravitational theory is, because we have become inured to it over the years. We don’t normally observe planetary motion with our own eyes, but if you watch a graphic representation of it on film it strikes you as a kind of bizarre celestial ballet, in which massive bodies dance around each other as if mesmerized. Planets and moons orbit other bodies without apparent influence, describing precise trajectories, as if of their own volition. An invisible hand seems to guide them. Newton tells us that the cause of this ballet is a force called “gravity”, but there doesn’t seem to be any forcinggoing on—no pushing and shoving. We are informed that the motions involved are proportional to the mass of the bodies (as well as their distance from each other), but why the quantity of matter in a body should affect its power of remote control is not explained (why not volume instead?). It’s all a lot to swallow. And nothing further is said to render things any clearer. On the face of it we are presented with celestial choreography that defies understanding, to which various labels have been applied by stipulation (“force”, “gravity”). It looks like a machine without any internal rationale or intelligible design–a happy confluence, a cosmic magic trick. Evidently there is some sort of attraction going on, but the nature of the attraction remains opaque.

Suppose you were interested in other sorts of motion, say the motion of leaves in the wind. You don’t know that the air is composed of gases with molecular components that make impacts on leaves invisibly. Some bright spark (a Mr. Newby) suggests that these motions are the result of a force emanating from another galaxy, which he chooses to designate as the “intergalactic force”: the force operates over vast distances instantaneously (or at the speed of light) and obeys an inverse square law. You might well be skeptical of such an extravagant theory, wondering if it is even intelligible. And Mr. Newby’s fancy mathematics, predictive though it may be, would not quell your anxieties. “How is that even possible?” you would think. The assertion that no other theory is in the offing (there is no theory of gases to appeal to) would not assuage your worries. But isn’t this essentially the predicament we are in with respect to Newton’s theory of gravity? A mysterious force operating over vast stretches of empty space, correlated de factowith mass, just seems like a preposterous speculation. It is no wonder that theorists of the time felt the need to introduce ether for it to operate through, in a halfhearted attempt to resuscitate mechanism: at least that would restore proximal causation. What if a follower of Mr. Newby (a Mr. Carter) sought to extend the theory to cover psychophysical interactions that involve immaterial substances: Mr. Carter believes that immaterial minds exist and that their motions are governed by the same force that applies to leaf movements etc. Clearly, he reasons, minds move around when bodies do, and this requires explanation—so Mr. Carter posits a force operating from afar that ensures such movements, thus preventing minds from being left behind when bodies move. Even followers of Mr. Newby might be skeptical of Mr. Carter’s radical departures from commonsense, but the latter ingeniously argues that such causation is no more occult than what is already accepted. It is simply a fact, he insists, that immaterial minds are also subject to the intergalactic force—the nature of the substance is irrelevant to its powers of motion under the influence of remote objects. Granted this is not intelligible to us, but neither is the prevailing theory of wind-driven motion—or planetary motion for that matter. After all, once you have accepted remote causation through a vacuum in your theory of planetary motion, why not extend this idea to other sorts of motion in the absence of anything better to say? And if unintelligibility is no objection, why not go the whole hog? Isn’t it simpler to accept that all causation is at-a-distance once it is has been admitted in one area? The only objection to it can be the existence of other better theories, not the intrinsic character of this theory. And it is surely true that Newton and company would leap at the chance to dispense with action-at-a-distance if they could. Accepting that theory is a burden not a joy. If only mechanism had worked out!

Now imagine some gigantic creatures marauding around the universe: they are bigger than a galaxy by quite a bit. Their senses are somewhat dull, so a galaxy like the Milky Way looks rather the way a golf ball looks to us. In fact, they use galaxies as pieces in their games. They are still tiny compared to the universe as a whole and know of another species ten times their size far away (fortunately). They are interested in the science of galaxies and form theories of them. Their theory is that galaxies are made of things called “stars” and “planets”, which move about according to laws. These objects they refer to as “particles” and they admit that no one has ever seen one of these invisible theoretical entities. Their epistemic relation to the giants is like the epistemic relation of atoms to us. At some point in their history a towering genius suggested that these particles move as a result of a force called “gravity”, which the said genius characterized much as Newton did. Let’s suppose they have never seen any movements governed by this mysterious force but accept it merely on theoretical grounds. Wouldn’t that theory strike some of them as pretty far-out? How can the particles move in relation to each other by dint of an occult influence operating over empty space? Mechanical influence they understand (they hit galactic golf balls all the time), but this postulated “gravity” business strikes them as baffling. It seems to them suspiciously like a type of telepathy. Surely the scientists can come up with something better than that! Admittedly, the universe is a strange place, and much remains mysterious, but it is hard to accept that matter is made of objects that affect each other at a distance without any mediating medium. Their reason recoils at the idea. Perhaps there are things the scientists are missing—maybe they lack the right concepts to make sense of reality. Their microphysics clearly needs an overhaul.

My purpose in rehashing and dramatizing these concerns is to draw an analogy to the classic mind-body problem. For the structure of both problems is remarkably similar: both involve an explanatory question and an array of options that fail to carry conviction. In virtue of what do things move? In virtue of what do things have minds? These questions are not easy to answer and they spark intense controversy. An extreme response is to deny that the problematic phenomena exist at all: there is no motion and there are no minds. These things are apparent but not real—as with Zeno and the eliminative behaviorists. There is nothing real to worry about. Then we have the reductionists: mind reduces to matter, and so does the power of motion. Mind is really nothing but a congeries of correlated brain states, and motive power is nothing but its correlated basis in objects—mass in the case of Newton’s physics, natural teleological tendencies in the case of Aristotle’s. Of course, the view is implausible in the extreme for motion, since the intrinsic properties of matter are precisely static(though they can be given a dispositional interpretation); but implausibility is not guaranteed to stop reductionists in their tracks (maybe motion is static too from a four-dimensional perspective).[2]Then we have dualism: motion is simply an additional dimension of reality, a brute fact that can’t be explained by anything else. Objects don’t move in virtueof anything in matter—as minds don’t think in virtue of anything in matter. In effect, Newton’s theory is dualist, since it invokes a force as an extra ingredient in the universe existing over and above such properties as extension, mass, solidity, shape, etc.[3]This theory might go along with supernatural infusions, with God serving as the prime mover keeping everything in motion—as he acts as the author of the mental world. Thus we have the idea of “subtle spirits” lurking inside bodies that ground their ability to move and to move other bodies (Newton looked for evidence of these but couldn’t find it). Physics itself turns out to be dualist. It is in virtue of non-material (non-mechanical) properties of material things that gravity works as it does—not in virtue of material properties like mass.

The ether theory operates conceptually rather like panpsychism: just as the mental is everywhere enabling mind to emerge from a material (sic) basis, so matter is everywhere (in attenuated form) enabling motion to “emerge” from spatially remote objects. Pan-materialism is the elixir that allows bodies to influence each other across space (compare Descartes’ plenum). Thus miraculous emergence is avoided and explanatory obligations discharged (allegedly). Mysterianism also has its counterpart: gravity (and hence motion) arises from matter because matter houses unknown properties that enable the apparent miracle to occur. True, matter as we conceive it cannot explain its capacity for motion—hence the eerie feeling we get when we contemplate planetary motion—but matter has a hidden nature that grounds its ability to move. Let’s call this hidden nature (relative to our cognitive capacities) G: then we can say that things move in virtue of G. G is an unknown quantity, possibly as a matter of principle for creatures like us. In effect, this was Newton’s position, since he thought that gravity had a cause, just a cause we can’t discover—or at least haven’t discovered by our most diligent efforts.[4]

Thus the motion-body problem tracks the mind-body problem. It has the same general form and generates the same array of theoretical options. This is not to say that the problems are identical—mind clearly involves features unique to it (subjectivity, intentionality, freedom, etc.): but the structureis much the same. In particular, the thrust towards accepting mystery is common to both: there is evidently something we are missing in both cases, something vital. This might be because of inherent cognitive limitations on our part—or it might be remediable by further theoretical effort (but don’t count on it). The problem of motion was the Great Problem of early modern science, and it has by no means been vanquished; likewise the problem of consciousness (or the mind in general, including the unconscious) is the Great Problem of our age. Both carry the whiff of mystery—I would say, the fact of mystery. We simply don’t know in virtue of what matter moves, as we don’t know in virtue of what matter thinks (feels, senses, etc.). We know that things do move, and we know quite a lot about the laws of motion, but we are clueless about the origin of motion.[5]So the world is not fully intelligible to us in this respect. As Locke says, other “spirits” might grasp the principles involved, but we are fumbling in the dark. It seems extremely doubtful that what explains motion will also explain mind (that would be a truly Unified Theory!), though I suppose nothing can be dogmatically ruled out; but what does seem true is that there are great gaps in our understanding of reality, as Locke conceded.

C

[1]See Noam Chomsky, What Kind of Creatures Are We?(Columbia University Press, 2016) for a detailed and illuminating discussion of the history of this issue (particularly chapter 4), which forms the background for what I say here.

[2]We might also consider non-reductive supervenience as a theory of the relation between movement and other properties of matter: if two material objects agree in all their non-dynamic properties, they must agree in their dynamic properties. That may be true, but it doesn’t answer the question of howmotion thus arises. Supervenience is not explanation. The same is true for mind-body supervenience.

[3]By contrast, Descartes is a monistic materialist about motion: he thinks it arises from mechanical causes. The so-called physical world is all of a piece for him—basically extension and collision (space itself is rarefied matter). Newton introduces a non-mechanistic element in the shape of gravitational force, thus finding an irreducible dualism at the heart of the physical world. Descartes’ mechanistic monism only falters when we come to mind, while Newton abandons materialism (mechanism) about the non-mental world. Newton, of course, had a strong interest in alchemy and other supernatural matters. We should speak of “Newtonian dualism” as well as “Cartesian dualism”.

[4]What if we claim that matter has an essentially mental nature, as with neutral monism? That would qualify as a hidden property of matter and hence could be the key to its attractive powers. But it is hard to see how this doctrine could make gravity any more intelligible, since “mental matter” would still need to operate across a vacuum at arbitrarily large distances. There seems noconception of matter, no matter how speculative or fanciful, that would render Newtonian gravity intelligible, at least to us.

[5]I have focused on gravitationally induced motion, but the same points carry over to electromagnetically induced motion: this too involves attractive (and repulsive) forces operating in a vacuum over empty space, equally violating mechanism. As Chomsky remarks, we have become used to abandoning world-directed intelligibility in favor of the lesser goal of theory-directed intelligibility (i.e. the theory is consistent and enables satisfactory predictions).