Is Matter Intelligible?
Matter has long been felt to be problematic. Berkeley thought it unintelligible, mainly on account of its causal inertness. Russell found it suspiciously akin to old-fashioned substance, replacing it with events and neutral (i.e. mental) stuff. The positivists suspected it of the crime of metaphysics and declared it meaningless. It is certainly not a perceptual given: we have no sense impression of matter. Please don’t say matter is what composes the objects of perception: that could be anything as far as perception is concerned—sense data, immaterial souls, God’s body. Matter is a postulate, a theoretical entity. As such it demands explication: what is it? Several ideas have suggested themselves: solidity is one, extension another. There are obvious problems with solidity: it is a dispositional not a categorical concept; it is not clear that all matter is solid; it is questionable whether only matter is solid (impenetrable)—what about minds and selves? Thus extension has tended to be the default position, following Descartes: the mind is not extended but matter always is (even if not invariably solid). This is the essence of matter, what makes it stand apart. The idea is encouraged by the phenomenology of perception, especially vision: for we see the world as composed of segmented objects of varying shapes and sizes—matter is simply the stuff that has these spatially defined properties. The mind doesn’t strike us this way, as extended in space, as consisting of variously shaped discrete objects. And extension is intelligible, as intelligible as geometry, so matter can be intelligibly defined. Moreover, this definition of matter fits nicely with mechanism as a causal thesis: objects cause changes in other objects by making contact at their boundaries (and not otherwise)—their extension is what fixes their sphere of operation. Extended bodies move about and collide with other extended bodies; and that’s how the world works, causally speaking. Matter, then, is the ideal stuff to constitute the subject matter of physics, because its essence—extension—is integral to causation. The natural world is made up of extended things interacting by contact causation. Notice that objects need well-defined boundaries for this conception to work; they need surfaces and geometric figure. They need to be as they appear in perception, more or less. The theoretical postulate must mirror perceptual phenomenology.
But now there is a problem: his name is Isaac Newton. Here we need to understand just how upsetting Newton was to Cartesian mechanistic physics; indeed I am going to suggest (following others) that Newton destroyed the concept of matter, by rendering it unintelligible. And not just Newton: the pioneers of electromagnetism drove the point home even more powerfully (James Clerk Maxwell has a lot to answer for). The problem, famously, has to do with action at a distance—causal powers exercised over empty space. There was already a problem about space: space has extension like matter, but space isn’t material. Descartes solved it by declaring space to be a form of matter, and relative views of space appeared to allow room for this type of maneuver (not to mention more modern views of space); so this objection looked to be surmountable, if disquieting. But the new problem was far more troubling because it questioned the foundations of mechanism. Objects no longer interact by contact at their boundaries: their mode of extension does not confine their causal powers. Bodies can influence other bodies that lie far beyond their boundaries and never make contact with them. That looks to spell the demise of mechanism (but see below), but it also throws into question the whole idea of material extension. For where now do bodies begin and end? What is the extension of the Sun, say? We already know that the Sun is in fiery flux at its surface and that it sends out particles of light across space, thus enlarging its sphere of operations; but with Newton we need to accept that it projects a force across space that has an impact on other bodies. Why isn’t this force part of the Sun? Just because we can’t see it doesn’t mean it isn’t there (imagine it appearing as colored in some way). Similarly, we now know that bodies exert electromagnetic force that projects beyond their surfaces (though diminishing quickly)—or do these forces really just enlarge the surfaces? Under mechanism we don’t need to worry about these questions, because bodies have no effects that go beyond their mode of extension; but with gravity and electromagnetism we have to reckon with effects that reach beyond what we are naturally inclined to call their boundaries.  This idea comes to seem arbitrary and hidebound: modern physics suggests that objects are not as confined in one place as we thought, and that their physics extends beyond what we can perceive as boundaries. Thus there were suggestions that bodies should be viewed as spread out in space according to their causal powers: the Sun is everywhere it exerts an influence! The gravitational and electromagnetic fields are parts of the object itself (as the earth’s atmosphere is part of the earth). This has the advantage of salvaging something of mechanism, because now we can say that objects exert causal power via the fields that surround (constitute) them: the fields “touch” remote (i.e. proximate to the fields) objects. We don’t think of the forces as extrinsic to the extended object, which makes things look causally occult; we think of the object as including the forces via the fields. In fact, we drop the idea of spatially enclosed objects with definite shapes and sizes hovering at some distance from each other, replacing it with the idea of spatially distributed fields with no clear boundaries and shapes. That is, we give up the idea of extension as a theoretically relevant property, as it was under pre-Newtonian Cartesian physics. In effect, this is the physics we find today, in which the concept of extension plays no significant role, not as classically defined anyway. The geometry is quite different, as is the picture of underlying reality. Perceptual phenomenology is no longer taken for granted as a guide to the objective nature of the physical world.
But this destroys the concept of matter. Matter is no longer a useful and viable concept for describing the world of physical interactions: it isn’t part of the physics of gravity and electromagnetism. The only (halfway) intelligible definition of matter, as extension, no longer applies to the subject matter of physics: there isn’t any matter in that sense (as there isn’t any phlogiston or entelechy). We can certainly allow for the “physical reality” of fields and forces, but these are not to be characterized in terms of classical extension, and hence are not “matter”; they are invisible unbounded spheres of influence of a rather elusive character. We have no commonsense ontology for fields and forces, as we do for extended objects under mechanism; physics has left our ordinary notions of material things behind. The object you see when you look at a cup, with its neat boundaries and definite shape, is not the object described by physics, which radiates out across space in virtue of its gravitational and electromagnetic powers. Just think of a simple magnet: its physical reality is not confined to the spot in space where the magnet appears to be. The reality consists of a rather mysterious force that acts on remote objects (remote according to ordinary perception, not remote from the point of view of the field of force involved). There is no matter in the sense ventured by traditional physics, simply because the commonsense notion of extension turns out not to apply to the physical world. Descartes was well aware that he was offering a theoretical edifice into which physical phenomena could be placed, but with later physics it turned out that this edifice had no application. Physics is not “the science of matter” in any intelligible sense, because there is none. Nor could there be once the reality of gravitational and electromagnetic forces is recognized. The concept derives from pre-scientific ideas rooted in the common sense associated with human perception; it is not part of the “absolute conception” propounded by physics. We could even say that the perception of extension in objects is a kind of sensory illusion—there is no such thing out there. It arises because fields and forces are invisible and because we have a biological need to segment the world into localized packets. We thus think that reality objectively comprises extended objects with clear boundaries, but that is projection encouraged by illusion. The world of Newton and Clerk Maxwell, not to mention quantum physics, is very different, better imagined as a kind of sea of overlapping fields and forces. It isn’t spherical billiard balls striking each other in the way the manifest image suggests; it’s shapeless fields of force invisibly distributed over space flowing into each other. Currents and smears, not nuggets and chunks: there is no matter to be found anywhere. 
 There is a subsidiary worry about gravity and matter if we define matter in terms of extension, namely that gravity does not vary with extension. It isn’t that the bigger or more spherical an object is the greater its gravitational attraction, yet the essence of matter is said to be extension. This is awkward for Descartes and the mechanists because the key property of matter (viz. extension) is not responsible for its most conspicuous effect (viz. gravity). Instead gravity is a function of mass, which is not definable in terms of extension. Instead mass is “quantity of matter”, which leaves us where we started: is mass the “quantity of extended things”? This difficulty might induce us to switch our definition of matter from extension to mass, but that too has problems, as Descartes was no doubt aware, which is why he doesn’t propose it. Definitions in terms of inertia are dispositional not categorical, and why should minds and selves not have inertia? And what about extended stuff that has no inertial mass? Mass by itself cannot define a workable notion of matter.
 The idea that according to modern physics there is no matter in the universe is not of course original to me; in fact, it is a commonplace of popular science. What I have done here is spell out the reasoning behind this verdict, with special reference to the concept of extension. Without this concept the word “matter” has no definite meaning, signifying something like “the stuff the world is made of”—whatever the nature of that stuff might be.