CLASSICAL ARGUMENTS OF EMPTINESS AND AETHER (Pt. 2 of 5: CARTESIAN VS. NEWTONIAN PHYSICS)

      In his 1644 Principles of Philosophy, Rene Descartes lent a tremendous amount of credibility to the Aristotelian or “plenist” side of the debate. He posed a multi-faceted natural philosophy that regarded space as a plenum or “aether”, and he backed up his assertions with firmer logic and broader explanatory power than his predecessors. He rejected void space using a unique argument that for a short time became very popular. In addition to his gripes that absolute space is functionally useless— it has no physical properties nor can any location ever be determined within it— he argued that the only fundamental characteristic we can apply to a body is its extension and that all other qualities of a particular body are not vital to its status as “body.” Body is not defined by hardness, for example, as we can imagine a body that might “recede as quickly as our hands approached.” Hardness and solidity are relative concepts that we apply to a body, and body is not defined by any relative description of its qualities. Likewise, color, temperature, and weight are not fundamental to the concept of a body either. A body— whether soft, translucent, hot or cold, heavy or lighter than flame— is still a body. Extension is the only fundamental and objective quality of a body. We apply extension to that which we call void space, and therefore it must be substantive.
      His next argument is highly reminiscent of the dialogue between Hermes Trismegistus and Asclepius. In both cases it is demonstrated that the notion of “empty space” is relative to that which we expect to find there. An “empty” jug may not contain water, but it does not contain void in any genuine sense. A particular region of space may not contain a “sensible object,” but this is not justification to say that the region is entirely without substance.
      Finally, Descartes objects to the core tenet of atomism: that matter is built of tiny, indivisible particles called atoms. He claims that there is no such thing as an indivisible particle, as the smallest object in existence can always be divided further, if not by hand then by thought. Every bit of matter is defined by its extension, and we can divide that extension— mentally if not physically— ad infinitum. He attributed the existence of these tiny particles to the vortex motion of his aether, and he attributed gravitation to the currents produced by these various vortices.

Descartes | Image Source

      Taken all together, Descartes’ argument is simple yet profound. He establishes with compelling logic that space is a body and body a space, that extension is not only a prerequisite of substance but an affirmation, and that indivisible atoms do not exist. Not everyone, however, finds Descartes’ argument as compelling as I do. Ian Hinckfuss, for example, in his 1975 book The Existence of Space and Time, writes:

He claimed, plausibly enough, that it is contradictory that there should be extension that is the extension of nothing, whence he concluded that since there is extension in supposedly empty space there must be substance there as well… But the conclusion does not follow. For an extra premise is needed— namely, that everything is substantial… That Descartes’ hidden premise is false is clear from the fact that we do not regard numbers, points, surfaces, and shapes as substantial objects— but they are all things to which we can make reference. Even if it were true that making reference to some or all of these things was systematically misleading, it would not follow that such a reference would imply the existence of something substantial. Measurements of mass are measurements of an amount of matter. Length, area, or even volume measurements are not.

      Quite clearly, Hinckfuss misinterprets Descartes’ argument. He discounts the argument using tangential and irrelevant evidence. He notes that “we do not regard numbers, points, surfaces, and shapes as substantial objects— but they are all things to which we can make reference.” This is true, but Descartes does not tell us that “anything to which we can make reference is a body.” If he had, we would also have to declare all nouns (beauty, perseverance, reference, et cetera) as substantial, which of course is absurd. One may say that numbers, surfaces, and shapes have extension, but they do not have the extension that Descartes says is necessary, namely “length, breadth, and depth.” And Descartes does not apply these terms to hypothetical objects, either. He does not say that an imaginary cylinder or an ideal sphere contain substance; he says that the three-dimensions of the real world contain substance. Hinckfuss seems to have confused Descartes’ conclusion with his premise. Descartes’ premise is not that everything is substantial. His premise is that a body is something which has extension, specifically a three-dimensional extension. Based on this premise, Descartes concludes validly: that which has extension is a body. The conclusion that everything three-dimensional is also substantial follows directly from our conclusion that “it is contradictory that there should be extension that is the extension of nothing,” which, as Hinckfuss admits, is plausible enough.
      But Ian Hinckfuss is of course not the only one to disagree with Descartes. In 1644, the same year that Descartes published his Principles of Philosophy, Evangelista Torricelli created a barometer, and seemingly a vacuum, by pouring mercury into a tube, and turning the tube upside down in a bowl of mercury. The mercury in the tube sank part of the way toward the bowl, creating a spooky, empty pocket above the suspended fluid. The pressure prevented the mercury from sinking all the way into the bowl, and there was no bubbly influx of air to explain the empty pocket. Torricelli, the “Hero of Alexandria” of the 1600’s, defied his contemporaries and dared to call this pocket a vacuum, as there was no reason to believe that particles of air filled the space, and no other explanation for what might exist there. The controversy of the vacuum claim cannot be understated. In the words of Alban Krailsheimer, “It is hard to think of any modern parallel to the shiver of horror engendered by the mere suggestion to a man of the seventeenth century that a vacuum could effortlessly exist and be maintained; a materialist forced to admit irrefutable evidence of life after death might offer a fair analogy.”

Torricelli | Image Source

      A man named Pierre Petit heard of Torricelli’s experiment and decided to try it himself, but the tube he used was too short and the desired result was not reproduced. In October 1646, Petit found himself traveling through Rouen, and he stopped for a visit at the home of Etienne Pascal. Upon hearing of the failed experiment, Etienne remarked that Rouen was home to one of the best glass works in Europe, and so Petit delayed his departure while a four foot tube was made for him. When the tube was finished, Petit performed the experiment in front of the entire Pascal family, including the brilliant son of Etienne, Blaise Pascal.
      Blaise was astonished and intrigued by the successful demonstration, and after Petit’s departure he began purchasing tubes and performing the experiment himself, convinced that a true void space had been created. In the summer of 1647, Blaise moved to Paris. His mathematical work and his apparent demonstration of vacuum had both earned him widespread notoriety, and, though quite ill, he received a variety of illustrious visitors to his bedside. In September, Descartes came to visit. Descartes, predictably, was resistant to the notion that Pascal had created a true vacuum by Torricelli’s method. He suggested that the top of the tube was filled with the “subtle matter” of the aether. Over the course of three meetings, tension developed between the two men, and there was no fourth meeting. Pascal resolved to prove his hypothesis by observing the effect of atmospheric pressure on the height of the fluid in the tube; he would carry his Torricellian vacuum up a mountain and monitor the fluctuations. Descartes claimed to have suggested this experiment during one of their meetings, a disputed point which further exacerbated the tension between the two thinkers. Due to his poor health, Pascal had to outsource the task to his brother-in-law, Florin Perier. He wrote to Perier: “If it happens that the height of the [mercury] is less at the top than at the base of the mountain… it follows of necessity that the weight and pressure of the air is the sole cause of this suspension of the quicksilver, and not the abhorrence of the vacuum: for it is quite certain that there is much more air that presses down at the foot of the mountain that at its summit.”

Pascal | Image Source

      As predicted, the mercury sank as Perier climbed into lower air pressure, and Pascal saw this as irrefutable evidence of a void in the space above the fluid. He concluded, “Until someone has shown me the existence of a matter which fills [the apparent void], my opinion will be that it is a real void, and destitute of all matter.” Pascal was apparently unconvinced by Descartes’ arguments regarding relative emptiness, and he also showed no concern with the light passing through his supposed void. At the time, many still supposed light to be an instantaneous phenomenon, involving neither waves nor particles, which were about to become the two dominant theories. Either theory would imply the presence of matter in the apparent void. If light were the result of wave phenomena, then a medium would have to exist in the gap to propagate the waves. If light were composed of particulate matter, that matter would be traversing the gap from every direction.
      In spite of the light problem, Pascal and his Torricelli vacuum validated the notion of void and heavily damaged the reputation of Cartesian physics. The stage was set for Sir Isaac Newton to finish the job. Newton faced off against Cartesian physics in a highly-publicized war of words, and in doing so ushered in a new era of deterministic, mathematical thought, which we call today “Classical Mechanics.” Newton first entered mainstream academia with his 1675 Hypothesis of Light, which countered the wave-based, Cartesian theory of light Robert Hooke had suggested in 1665. Hooke suggested that light was a result of high-frequency, ethereal oscillations, and that refraction resulted from the disruption of these waves. Newton rejected Hooke’s theory of light, claiming that refraction could only be explained if light rays were particles endowed with inherent size and color.

Newton | Image Source

      The optical debate ran parallel to a broader battle between Cartesian physics and Newtonian atomism. Newton believed in “absolute space” as something separate from physical matter. Newton submitted to Descartes’ logic that that which has extension cannot be nothing, but he refused to say that this extension is body. He wrote: “…although space may be empty of body, nevertheless it is not in itself a void; and something is there because spaces are there, although nothing more than that.” Clearly, it is paradoxical to say that space is simultaneously something and yet empty of body. Over the course of his career, Newton would continue to refer to his absolute space in a variety of confusing and contradictory terms. For example:

[God] endures always and is present everywhere, and by existing always and everywhere he constitutes duration and space. Since each and every particle of space is always, and each and every indivisible moment of duration is everywhere, certainly the maker and lord of all things will not be never or nowhere … God is one and the same God always and everywhere. He is omnipresent not only virtually but also substantially; for active power cannot subsist without substance.

      Newton claims that space is distinct from body, but he says that it is also substantial by virtue of God’s omnipresence and “active power.” Elsewhere Newton writes, “Absolute space, in its own nature, without relation to anything external, remains always similar and unmovable.” How can something with “active power” be “always similar and unmovable?” Perhaps a similar concern is what prompted Newton’s contemporary, Gottfried Leibniz, to joke that Newton regarded space as “an organ which God makes use of to perceive things by.”
In a famous correspondence between Gottfried Leibniz, Newton, and Samuel Clarke, Leibniz highlighted a variety of issues with Newton’s theory. He said that a substance has an intrinsic principle of action, and “if space is a property or attribute, it must be the property of some substance.” Without a property, attribute, or “principle of action,” absolute space would be useless, and there would be no reason to posit its existence. At the same time, one cannot logically apply an attribute or principle of action to something that is considered “void.” Samuel Clarke, who tended to act as Newton’s wingman, responded: “Void space is not an attribute without a subject; because, by void space, we never mean space void of everything, but void of body only. In all void space, God is certainly present.” For Newton and Clarke, it would seem, bodies are like ulcers upon the still, ethereal spirit of God.
      The primary reason Newton and Clarke supported the seemingly useless idea of an absolute space is that it appeared to them a necessary prerequisite of inertia and of “determinate motion.” While Newton’s philosophy was prone to contradiction, he was a genius of practical science. He refused to embrace the Cartesian idea of relative motion on the basis that it undermined his ability to determine an absolute velocity and location. The logic of his position is patently circular, but it is necessary for his mathematical goals. Take, for example, Newton’s De Gravitatione. After defining Descartes’ philosophical position, he essentially argues, “This cannot be true, because then all my predictions would be false.”
      Judge for yourself:

      …that the absurdity of this position may be disclosed in full measure, I say that thence it follows that a moving body has no determinate velocity and no definite line in which it moves. And, what is worse, that the velocity of a body moving without resistance cannot be said to be uniform, nor the line said to be straight in which its motion is accomplished. On the contrary, there cannot be motion since there can be no motion without a certain velocity and determination.
      But that this may be clear, it is first of all to be shown that when a certain motion is finished it is impossible, according to Descartes, to assign a place in which the body was at the beginning of the motion; it cannot be said whence the body moved. And the reason is that according to Descartes the place cannot be defined or assigned except by the position of the surrounding bodies, and after the completion of a certain motion the position of the surrounding bodies no longer stays the same as it was before. For example, if the place of the planet Jupiter a year ago be sought, by what reason, I ask, can the Cartesian philosopher define it? Not by the positions of the particles of the fluid matter, for the positions of these particles have greatly changed since a year ago… Truly there are no bodies in the world whose relative positions remain unchanged with the passage of time, and certainly none which do not move in the Cartesian sense: that is, which are neither transported from the vicinity of contiguous bodies nor are parts of other bodies so transferred. And thus there is no basis from which we can at the present pick out a place which was in the past, or say that such a place is any longer discoverable in nature.

      Every conclusion that Newton makes here is valid and eloquently stated— but to what end? He has not disproved Descartes’ notion of space— he has merely elucidated its problematic philosophical consequences, consequences which undermine the notions of “determinate space” and “determinate velocity.” Newton hypothesizes an absolute space so that all objects can be shown to travel at a fixed speed in a fixed direction relative to that space, but Cartesian relativity cannot be shown false on the basis that it does not support these mathematical goals.
      Strangely, however, Newton seemed to occasionally endorse a Cartesian plenum in lieu of a vacuum. He wrote a letter to mathematician Richard Bentley in response to the charge that his physics had made gravity an “innate, inherent, and essential” quality of matter, unmediated by contiguous forces. Newton writes:

That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great of an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking, can ever fall into it.

      It is clearly ironic, then, that Newton’s physics provided the foundation for “so great of an absurdity” for centuries thereafter.

TO BE CONTINUED

Thank you for reading! If you enjoyed this article, look out for part three, where I'll be talking about competing theories of light and optics and how they pertain to the larger debate between plenist and atomist philosophy!

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