Discussions about ethics and the kind of public policy that should follow from sound ethical principles can appear to be endless when there is fundamental disagreement about the first principles from which such discourse proceeds. When we do not recognize these underlying differences in debates about abortion, embryo-destructive research, same-sex marriage, the role of government in human affairs, indeed the very nature of what it means to be human, they will only find us, along with the fallen angels in Milton’s Paradise Lost, “in wandering mazes lost.” Not only debates about key social issues, but also those about fundamental economic policy, are never properly joined if interlocutors proceed from initial assumptions that are wildly diverse.

As Aristotle observed, in order to have fruitful discourse we need to make sure that we really are talking about the same thing in the same respect. For example, those who appeal to various forms of natural-law ethics presuppose both the intelligibility of the very notion of nature and that any sound ethical theory must find its roots in the natural order. In this tradition, nature discloses an inherent intelligibility and purposefulness that, in principle, human reason can discover.

It is, however, precisely the notion of nature and human nature so central to the tradition of natural law ethics that is rejected by many who label themselves “progressives.” Instead, they often embrace a mechanistic view of the world, and a reductionist materialism that denies any inherent purpose in nature, because they see them as necessary correlatives to the rise of modern science. Even if they are dissatisfied with mechanism and materialism, they think that appeals to the traditional understanding of nature and human nature are hopelessly misguided, the product of a kind of nostalgia for a past that the modern world has irrevocably left behind. If one wishes to argue seriously for a traditional understanding of what it means to be human, one must at least recognize (if not confront) a common, often unstated commitment to the view that the modern world, especially modern science, has rendered irrelevant the very principles underpinning any such understanding.

It is easy for us to accept as incontrovertibly true a master narrative of modernity according to which our modern world is born (at least in part) in the Scientific Revolution of the 17th Century. In that revolution, modern science emerged, versions of this grand interpretation would have it, precisely by rejecting the conceptions of nature and human nature that are at the core of an Aristotelian-Thomistic world-view. Since there is a deep gulf, a fundamental incompatibility, between an Aristotelian notion of nature and the understanding of nature ushered in by modern science, any appeal to what Aristotle or Thomas Aquinas would say about the natural sciences is deemed irrelevant to the intellectual projects of modernity.

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The standard version of this narrative tells how Galileo and Newton overturned the antiquated Aristotelian view of the universe and established the principles of modern science. Although the master narrative has many subdivisions, it is useful to focus on what is regarded as the very first principle of the new physics, the principle of inertia: “every body perseveres in its state of rest or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.” Alfred North Whitehead called the principle of inertia “the first article of the creed of science, and like the Church’s creed it is more than a mere statement of belief; it is a paean of triumph over defeated heretics.” The defeated heretics, Whitehead explains, are “the Aristotelians who for two thousand years imposed on dynamics the search for a physical cause of motion.” According to this interpretation, Newton’s principle of inertia contradicts Aristotle’s principle that everything which is moved is moved by another. If Newton is right, Aristotle must be wrong.

Scholars often see significant consequences of the new science. Oxford philosopher Anthony Kenny, for example, denies the probative force of Aquinas’s proofs for the existence of God because these proofs are based on the principles of Aristotelian science, a science that the modern world has shown to be false. More importantly, for our purposes, Kenny also thinks Aquinas’s ethical theory suffers because Aquinas’s account of appetites in man and in animals depends upon a falsified “archaic physics.” Any notion of the natural agency of inanimate matter cannot be reconciled with the principle of inertia. Newtonian mechanics, according to Kenny, rules out any appeal to a teleological account of all of nature: “The operations of the laws of inertia and gravity and the natural activities of sulphur or uranium are not teleological activities at all. If we today are to seek, as Aquinas did, to locate animal desire and human willing in a hierarchy of different kinds of tendencies toward good, then we must put at the bottom level of the hierarchy not the natural agency of inanimate matter, but the non-conscious teleological activities to be found in the plant world.”

Kenny observes that, for Aquinas, “all action, including the most elemental actions of completely inanimate bodies, was . . . fundamentally teleological. This part of Aquinas’s system is something which must be discarded if we are to make any use of his philosophy at the present time.” Alasdair MacIntyre, despite his sympathies for Aristotle and Aquinas, argued, at least in his early writings, for the necessity of providing a new foundation for ethics, different from the natural philosophy of Aristotle and Aquinas. The new science, he argued, was a classic case of “systematically different and incomparable observational languages, key concepts, and theoretical structures [which] were framed in terms of rival and incomparable standards . . . [such that] there was no shared common measure [between the sciences of Aristotle and Newton].”

Hans Blumenberg and Wolfhart Pannenberg have argued that the principle of inertia lies behind a radical affirmation of the autonomy of the natural world. Just as motion needs no continuing cause for its existence, so too existence itself is a kind of given (at best only given in some initial creative act). As Pannenberg notes, it was Newton’s understanding of inertia in terms of a force that is inherent in bodies, along with the reduction of force to a body and its mass, that contributed in a decisive way “in the course of the eighteenth century to the removal of God from the explanation of nature.” And Blumenberg observes: “The modern age has regarded self-preservation (conservatio sui) as a fundamental category of everything in existence and has found this borne out all the way from the principle of inertia in physics to the biological structure of drives and the laws of state building.” Self-assertion and human autonomy are the broader cultural correlatives of a fundamentally new principle of explanation—and, for Blumenberg, it is the principle of inertia that lies at the core of this new view of things.

When Blumenberg and Pannenberg expand the implications of the principle of inertia to the realm of creation, they exhibit a deficient view of the kind of metaphysical dependency the doctrine of creation entails. To think that the continuing reality of any motion or change or of a state of rest does not need an explanation in terms of a cause—which is precisely the interpretation given to Newton’s principle of inertia—does not mean that the continuing existence of all that is, as it exists, in whatever way it exists, does not require the causality of God as Creator. Explanations in the natural sciences, however they are understood, do not extend to fundamental questions of existence, which belong, rather, to the domain of metaphysics. Whatever self-sufficiency one might find in nature, it is not an absolute self-sufficiency; it is only a self-sufficiency with respect to principles of the natural sciences, but not with respect to God as cause of all that is. Changes in the world can be explained in terms of causes found in nature. Such explanation is the proper domain of the natural sciences. But these causes are not sufficient to explain why things exist in the first place: why there is something rather than nothing.

What about the principle of inertia and explanations in the natural sciences, especially as these explanations entail notions of nature central to ethical reflections? Newton’s famous three laws of motion appear early in the Principia. He begins with a series of definitions concerning mass, forces, and the like; then states the three laws and deduces corollaries from them. These laws, corollaries, and definitions are expressed in expository prose without those mathematical equations with which we have become familiar. In the preface, Newton reminds the reader that in this work he will “subject the phenomena of nature to the laws of mathematics.” At another place, he observes that he only provides “a mathematical notion of . . . forces, without considering their physical causes.” He tells us that he is considering “forces not physically, but mathematically.” At the beginning of Book III, the section called the “System of the World,” Newton notes: “In the preceding books, I have laid down principles not philosophical but mathematical.”

Immediately prior to his enunciation of the principle of inertia, Newton hypothesizes what would happen in projectile motion “if the resistance of the air is taken away.” In this discussion Newton moves from the world of ordinary experience to imagine the limiting case of motion: viz., that at which the projectile would proceed in its motion forever. Newton describes what obtains in a limiting case, and, thus, he presupposes the concept of limit in the derivation of that case. Newton’s notion of limit is drawn from mathematical modes of reasoning. It is precisely such a mathematical concept of limit which is at the root of the principle of inertia. The principle is an inference drawn from the mathematical-physical approach to a limit. In other words, as the resistance of the medium approaches zero, the distance traveled approaches infinity.

As an idealized, quasi-mathematical concept, the principle of inertia involves an abstraction from the extrinsic forces acting on real bodies moving in a physical environment. The principle also involves an abstraction from any kind of physical causality. Accordingly, the principle of inertia, regardless of its truth, says nothing in support of, nor in contradiction to, Aristotle’s principle in physics that everything that is moved is moved by another. The principle of inertia considers bodies simply as three-dimensional realities devoid of natures. Such an abstraction from nature is possible and appropriate in the mathematical study of nature. Remember the title of Newton’s work: The Mathematical Principles of Natural Philosophy [Philosophiae Naturalis Principia Mathematica]. Only if one assumes that the principle of inertia is a law of nature and not simply a principle in mathematical physics would one have a problem with its relationship to Aristotelian physics. The application of mathematics to the study of motion necessarily involves an abstract world. And an abstract world is not a false world; but neither is it identical with the world of nature.

Aristotle’s notion of nature, central to his physics and foundational for traditional ethics, is not challenged by the mathematical physics of Newton. Aristotle may be right or wrong on what nature and motion are, but he is not wrong because Newton is right. Newton and others developed a greatly expanded notion of the role of mathematics in the study of nature, but these “new sciences” do not really challenge the principles of Aristotelian physics. Much more needs to be said here about the relationship between mathematical physics and the physics of Aristotle (including the inadequacy of any number of conclusions within Aristotle’s physics). My fundamental point is that the rise of modern science, exemplified by the principle of inertia and the whole of Newtonian mechanics, does not require that we abandon the traditional understanding of nature.

The philosophical commitment to a mechanistic and materialistic natural philosophy is the product of philosophical traditions in the seventeenth century and beyond. Mechanism and materialism represent a radical rejection of Aristotelian and Thomistic natural philosophy, but mechanism and materialism remain excess baggage, not required in order to accept the advances in our understanding of the world that are the legacy of Galileo and Newton.

Distinguishing developments in mathematical physics from the philosophical baggage often associated with the Scientific Revolution allows us to avoid the error of thinking that the rise and development of modern science renders the philosophical insights of Aristotle, Aquinas, and others useless. Modern science does not require us to abandon notions of nature and human nature upon which so much of traditional ethics depends. A better understanding of the limits and inadequacies of the master narrative of modernity can not only help us to recognize the true source of the view that modern science is incompatible with the philosophy of Aristotle and Aquinas, but can also provide the kind of intellectual space necessary to take that philosophy seriously.