Digressions&Impressions

On Newton's Refutation of the Mechanical Philosophy

In the recent philosophical reception of Newton there is an understandable tendency to focus on the inverse square law of universal gravitation. I don't mean to suggest this is the only such focus; arguably his views on Space have shaped -- through the good works of Stein and Earman -- also debates over spacetime theories. 

The effect of this telescoping has also impacted, I think, the way in which the debate between the mechanical and Newtonian philosophy has been understood. The former is said to posit a contact model in which contact between very small corpuscles explains a lot of observed phenomena. A typical mechanical philosopher creates a hypothetical model, a machine with pulleys and levers (etc.), that can make observed phenomena intelligible. In the mechanical philosophy, which itself was directed against a variety of Aristotelian and Scholastic projects, efficient causation -- once one of four canonical causes (including formal, final, and material) -- has achieved a privileged status.

The scholarly fascination with the status of action at a distance is, thus, readily explicable because it violates the very model of intelligibility taken for granted in the mechanical philosophy. As Newton notes in the General Scholium (first published in the 1713 second edition of the Principia), universal gravity "operates, not according to the quantity of the surfaces of the particles upon which it acts, (as mechanical causes use to do,) but according to the quantity of the solid matter which they contain, and propagates its virtue on all sides, to immense distances, decreasing always in the duplicate proportion of the distances." 

Before I get to the main point of today's post, I offer two asides. First, with its emphasis on hypothetical explanations, the mechanical philosophers (and here I use the term to cover people as diverse as Beeckman, Descartes, Boyle, and Huygens) also exhibit a deep strain of skepticism about the very possibility of truly grasping nature's innards as it were. Spinoza's natura naturans and even Kant's ding-an-sich are the enduring expressions of this strain of skepticism (allowing that Kant is much less a mechanical philosopher). To put this as a serious joke: the PSR is, thus, not an act of intellectual hubris, but a self-limitation of the knower when it comes to fundamental ontology. Second, by showing that there is something wholly unintelligible about the way motion is supposed to be transferred from one body to the other (Essay 2.23.28), Locke, who gets so little credit among contemporary philosophers, had already imploded the pretensions of the mechanical philosophy on conceptual grounds.  Okay, so much for set up.

The mechanical philosophers were not so naïve to think that models that relied on mere impulse, matter in motion, could create hypothetical models of sufficient complexity to provide hypothetical explanations of the phenomena. This is especially a problem because the mechanical philosophers posited a homogeneous matter. So that in addition to matter and motion, they posited size and shape not merely as effects of motion, but also as key explanatory factors in the hypothetical models of visible phenomena (this can be seen in Descartes, Gassendi, and Boyle, whose "The Origin of Forms and Qualities according to the Corpuscular Philosophy" (1666), I take as a canonical statement of the mechanical philosophy). So that the mechanical philosophy is committed to privileging (to echo a felicitous phrase by Biener and Smeenk [here; and here]) geometric features of bodies.

Even leaving aside the inverse square law and its universal scope, Newton's experimental work on gravity demolished a key feature of the mechanical philosophy: size and shape are irrelevant to understand gravity. I quote from Henry Pemberton's View of Sir Isaac Newton's Philosophy (1728):

It will be proper in this place to observe concerning the power of gravity, that its force upon any body does not at all depend upon the shape of the body; but that it continues constantly the same without any variation in the same body, whatever change be made in the figure of the body: and if the body be divided into any number of pieces, all those pieces shall weigh just the same, as they did, when united together in one body: and if the body be of a uniform contexture, the weight of each piece will be proportional to its bulk. This has given reason to conclude, that the power of gravity acts upon bodies in proportion to the quantity of matter in them. Whence it should follow, that all bodies must fall from equal heights in the same space of time. And as we evidently see the contrary in feathers and such like substances, which fall very slowly in comparison of more solid bodies; it is reasonable to suppose, that some other cause concurs to make so manifest a difference. This cause has been found by particular experiments to be the air. --1.2.24 [emphasis added]

Pemberton (who was the editor of the third, 1726 edition of the Principia) goes on to give Boyle's famous vacuum experiments with falling feathers and stones as evidence for this argument. That is, Pemberton uses Boyle's experimental work to refute Boyle's mechanical philosophy. 

Now, in the Principia, references to Boyle's experiment got added only to the (1713) second edition in two highly prominent places: Cotes added a reference to it in his editor's introduction and Newton added a reference to it in the General Scholium at the end of the book. In both cases Boyle's experiment is used as a kind of illustration for the claim that without air resistance falling bodies are equally accelerated and for the plausibility of positing an interstellar vacuum. That is, if one reads the Principia superficially (by looking at prominent material at the front and end), it seems as if Newton and Boyle have converging natural philosophies.

Of course, neither Pemberton nor Newton rely exclusively on Boyle's vacuum experiment to make the point that shape and size (or geometry) is not a significant causal factor when it comes to gravity. The key work is done by pendulum experiments with different metals. (These can be found in Book II of the Principia, which is often skipped, although he drives the point home in Book III, Prop. 6 of Principia.) These show that quantity of matter is more fundamental than shape. And, crucially, shape & size and quantity of matter need not be proportional to or proxies of each other. This fact was by no means obvious, and at the start of the Principia. even Newton offers, as Biener and Smeenk have highlighted, a kind of geometric conception of quantity of matter in his first definition before suggesting that 'quantity of matter' is proportional to weight (and indicating his pendulum experiments as evidence thereof).

Let me wrap up. What's important here is that even if Newton had been wrong about the universal nature of the inverse square law, he showed that the mechanical philosophy cannot account for the experimentally demonstrated features of terrestrial (and planetary) gravity. (So, that the mechanical philosophy is not a natural way to understand Galilean fall.) And this means that in addition to Locke's conceptual claim, Newton shows that the mechanical philosophy's emphasis on just one kind of efficient causation, by way of contact, is not sufficient to explain the system of nature. 

What I say here is not surprising to students of Newton. But it's also not really much emphasized. To be sure, Newton, too, accepted a kind of homogeneous matter, but rather than its size and figure, he showed that an abstract quantity (mass) is more salient. Of course, how to understand mass in Newton's philosophy opens new questions, for as Ori Belkind has argued it should not be taken as a property of matter, but rather as a measure.