This image of the Solar System (with the Sun at S) comes from René Descartes’ enormously influential book, The Principles of Philosophy (1644). Offering mechanistic accounts of several natural phenomena, Descartes believed that the whole universe was completely full of matter, with no truly empty spaces and no attractive or repulsive forces. He hypothesized that the Solar System is a great “tourbillon” (whirlpool), in which the tiniest particles of matter circulate around the Sun, dragging the Earth and the other planets around in their orbits—just as leaves might spin in the eddies formed when water flows past pillars (to offer an analog of my own). Descartes did not invent the mechanical philosophy, and other natural philosophers offered their own versions of it, but they all sought to explain things in terms of particles of matter that move and interact in predictable ways. Boyle promoted his own version of the mechanical philosophy in several experimental and theoretical works.
Boyle and the Mechanical Philosophy
During the Scientific Revolution, no idea was more influential—or more important for the future of science—than the “mechanical philosophy.” Mechanical philosophers conceived of nature as a great machine, an intelligently constructed system of unintelligent matter in motion rather than a living organism with a “soul” or “intelligence” of its own. No one did more to advocate for the mechanical philosophy—and to explore its theological dimensions—than Robert Boyle. The magnitude of his enthusiasm for it is best captured by this fact: when he published a treatise on The Excellency of Theology, Compar’d with Natural Philosophy (1674), it was coupled with a second work devoted to “the Excellency and Grounds Of the Corpuscular Or Mechanical Philosophy.”
The mechanical philosophy was essentially a modified version of ideas originally put forth by the ancient philosophers Leucippus, Democritus, and Epicurus. As they had presented it, atomism came with strong overtones of atheism: all things are composed of eternal, uncreated, invisibly small, indivisible particles, called “atoms” because they cannot be “cut’ into smaller pieces. The atoms move randomly and without purpose in an infinite void, bumping into one another to form macroscopic objects, including living things.
During the Scientific Revolution, however, atoms mostly lost their irreligious image, as Pierre Gassendi and others “baptized” atomism. Their general approach was to postulate atoms as divinely created particles of inert matter, with properties and powers given to them by God, who guided their motions to form larger bodies. In 1659, Boyle’s friend Henry More called this idea the “Mechanick philosophy,” and two years later Boyle himself called it “the Mechanical Hypothesis or Philosophy,” marking the first use of that exact term that I am aware of (Certain Physiological Essays, in The Works of Robert Boyle, vol. 2, p. 87).
The Christianization of atomism was a crucial step, for it enabled the adoption of a new worldview—a complete change in the way in which nature was conceived. For nearly two thousand years, the prevailing concept of nature came not from the atomists, but from Aristotle and Galen. This notion depicted “Nature” as a wise, benevolent being, capable of acting almost like a conscious agent, as seen in phrases such as “Nature does nothing in vain,” “Nature abhors a vacuum,” or “Nature is the wisest physician.” By contrast, for Boyle and other mechanical philosophers, the world was a vast, impersonal machine, incapable of acting consciously. Perhaps surprisingly, Christian theological beliefs helped drive this enormous conceptual shift that lies at the heart of modern science, in ways that historians have not fully appreciated until fairly recently.
Boyle was a crucial player in this important process. He believed that the traditional notion of “Nature” was both theologically and scientifically unacceptable. I will say more about the theological part in future columns. For now, let’s focus on the scientific part.
As Boyle saw it, Aristotle’s system suffered from a fundamental conceptual problem that had to be eliminated before scientific progress could be made. It implicitly gave matter the ability to think: how else (to offer an example of my own) could a body return to its “natural place,” unless it knew that it had arrived there? Properly speaking, Boyle argued, matter is utterly incapable of obeying “laws” (a term Boyle nevertheless employed often), because it lacks innate intelligence and cannot know whether or not it has obeyed. Likewise, when we use “such Phrases, as, thatNature …, or Suction, doth this or that,” we “ascribe to a notional thing, that which, indeed, is perform’d by realAgents; as, when we say, that the Law punishes Murder with Death, that it protects the Innocent, releases a Debtor out of Prison, when he has satisfied his Creditors (and the Ministers of Justice) on which, or the like occasions, we may justly say, That ’tis plain that the Law, which, being in it self a dead Letter, is but a notional Rule, [and] cannot, in a Physical sense, be said to perform these things.” What really does perform things in nature, according to Boyle, is “those Powers, which [God] gave the Parts of Matter, to transmit their Motion thus and thus to one another” (A Free Enquiry into the Vulgarly Reciev’d Notion of Nature, in The Works of Robert Boyle, vol. 10, pp. 464 and 457). Here we have the mechanical philosophy in a nutshell.
Aristotelian philosophy also explained many aspects of nature in terms of immaterial “forms and qualities” inherent to bodies. Boyle and other mechanical philosophers found such analyses vague and incapable of providing convincing explanations. As the prominent Anglican cleric Simon Patrick frankly observed, “To them that have once tasted of the Mechanicall Philosophy, formes and qualities are like to give … little satisfaction” (A brief account of the new sect of latitude-men: together with some reflections upon the new philosophy, 1662, p. 22).
In the experiment of Torricelli, a glass tube filled with liquid mercury (on the right as shown here) is inverted and immersed in a bath of mercury. The liquid drops until the pressure of mercury within it balances the pressure of the air outside the tube. The space in the tube above the mercury appears to be empty, and (as Boyle and others showed) it behaves as if it were—but this was a controversial claim at the time. Torricelli’s experiment is the basis for the “barometer,” a term that Boyle coined in 1663. When I taught high school chemistry many years ago, I used to demonstrate this experiment in class, something that is no longer permitted for environmental reasons. To see it done, watch this video.
To see why Boyle regarded mechanical explanations as superior, consider the maxim that “Nature abhors a vacuum.” Boyle flatly rejected that idea. “When I consider, how great a Power the School Philosophers [i.e., followers of Aristotle] ascribe to Nature, I am the less inclin’d to think, that Her abhorrence of a Vacuum is so great, as they believ’d” (A Free Enquiry into the Vulgarly Reciev’d Notion of Nature, in The Works of Robert Boyle, vol. 10, p. 536). Drawing on dozens of experiments he had carried out with an air pump, Boyle concluded that for practical purposes one could indeed create a vacuum, even though (in his opinion) the formal philosophical question of the actual existence of a space completely empty of all matter could not be settled with certainty. The ultimate inspiration for many of his experiments came from an experiment carried out in 1644 by Vincenzio Viviani, a former assistant of Galileo who was a student of Evangelista Torricelli. (See the caption to the image for more information.)
As Boyle observed, “when the Torricellian Experiment is made, though it cannot, perhaps, be cogently prov’d, … that, in the upper Part of the Tube, deserted by the Quick-Silver [mercury], there is a Vacuum in the strict Philosophical Sense of the Word; yet, … ’twill to a heedful Peruser appear very hard for [followers of Aristotle] to shew, that there is not One in that Tube.” Considering “the Space deserted by the Quick-silver at the top of the Pipe, … One may be Invited to doubt, Whether a Vacuum ought to be thought so formidable a Thing to Nature, as they imagine She does, and ought to, think It?” (A Free Enquiry, in The Works of Robert Boyle, vol. 10, pp. 536-7)
To test the idea that “Nature abhors a vacuum,” Boyle used the apparatus depicted on the right in this plate, from A Continuation of New Experiments Physico-Mechanical, Touching the Spring and Weight of the Air, and their Effects (Oxford, 1669). Because he typically provided readers with highly detailed, very carefully written descriptions of his experiments and the practical difficulties he had to overcome—in this instance, the account runs to four pull pages supplemented by the figure—they could more easily be replicated, lending credibility to his conclusions.
Furthermore, the strength of the vacuum—corresponding to the height of the mercury in the tube—depended only on the pressure of the atmosphere, not the ability of an imaginary being called “Nature” to prevent it. To investigate “the old Doctrine of the Schools, which would have Water raiseable in Pumps to any height, ob fugam vacui [to avoid a vacuum],” Boyle mounted a long metal pipe on the side of a house “about 30 foot high,” with a reservoir of water on the ground and a transparent glass tube at the top in order to observe the level reached by the water as the pump was operated. (See the figure.) When “the height of the Cylinder of Water was measur’d,” it came to 33½ feet, but no higher— regardless of how hard the pump was worked. At that point, “I return’d to my lodging, which was not far off, to look upon the Baroscope [barometer], to be informed of the present weight of the Atmosphere, which I found to be but moderate, the Quick-silver standing at 29 inches, and between 2 and 3 eights of an inch.” Boyle knew that mercury is about 13½ times the density of water. Thus, he concluded that “the difference between the height of the Mercury sustain’d by the weight of the Atmosphere in the Baroscope, and that of the Water rais’d and sustain’d by the Pressure of the same Atmosphere in the long Tube did not appear to differ more than an Inch or two from the proportion they ought to have had, according to the difference of their specifick Gravities” (A Continuation of New Experiments, in The Works of Robert Boyle, vol. 6, pp. 70-73).
Boyle interpreted such experiments in terms of the mechanical philosophy. Instead of saying vaguely that “Nature abhors a vacuum,” which implied that water and mercury could be raised to any height by a pump, he said more precisely that “the weight of the Atmosphere” balanced the weight of the water in the metal pipe and the weight of the mercury in the glass tube. This explained precisely why water and mercury rose to specific, quite different heights.
Altogether, we might say that Boyle abhorred the Aristotelian notion of “Nature” much more than “Nature” abhors a vacuum.
My next column uses additional excerpts from Boyle’s great treatise about God and nature to see just why he found the traditional notion of an intelligent “Nature” so deficient theologically—only adding to his enthusiasm for the mechanical philosophy.