During the Scientific Revolution, traditional views of human nature underwent a radical transformation. Thinkers like Francis Bacon and René Descartes introduced a more materialistic and mechanistic understanding of what it means to be human, moving away from religious and metaphysical frameworks. Thinkers began to view humans as part of the natural world, functioning according to laws and principles similar to machines.
In this lecture, we’ll explore how these early materialist ideas developed into what we now call the mechanistic approach to human nature. By understanding humans as rational, functional beings, it became possible to ask deeper questions about ethics: What is good or bad for human beings? How should we live? This is where natural law theories come in—offering a moral framework rooted in human nature itself.
One of the most significant transitions of this period was the evolution of natural law theory.During the medieval period, especially in the works of Thomas Aquinas, thinkers closely linked natural law to divine law. They saw law as an expression of God’s eternal order—something reason could discover, but ultimately grounded in a theological framework.
As Europe moved into the early modern era, this understanding shifted. Thinkers began to see law less as an external command from divine authority and more as a principle rooted in nature, accessible through human reason alone.Hugo Grotius was one of the first—and arguably the most influential—thinkers to articulate this secular version of natural law.
Theology and Reason in Aquinas and Grotius
Born in 1583 in the United Provinces of Holland and Zeeland, Grotius was a prodigious intellect. He entered the University of Leiden at the age of eleven and excelled in classical studies, particularly Greek and Roman literature. At fifteen, he joined a diplomatic mission to France, where King Henry IV famously called him “the miracle of Holland.”
Despite his early promise, Grotius became embroiled in the political and religious turmoil of his time. His opposition to the dominant orthodox Calvinist faction led to a life sentence in 1619. In a dramatic turn of events, he escaped from prison two years later, smuggled out in a book chest, and fled to Paris. There, he spent the rest of his life in exile, continuing his legal and philosophical work.
Knud Haakonssen, in his analysis of this intellectual shift, explains how Grotius redefined the concept of jus (right). Traditionally, in Roman law and in Aquinas, jus referred to what was in accordance with law. Grotius redefined jus as something individuals possess, rather than something that describes whether an action or condition aligns with the law. As Knud Haakonssen explains, Grotius—drawing on scholastic thinkers, especially the Spanish neo-Thomists—shifted jus from being a quality of actions to a personal right (Haakonssen, 1985, p. 240).
This redefinition marks a foundational change in political thought. Jus becomes a personal entitlement rather than a description of legal or moral order. It is a shift from a communal, divine framework to one centered on the individual. As Haakonssen further notes, “Whether it is set within Hobbesian threat of natural disorder or within Grotian promise of an ideal natural order… the history of political thought in the 17th and 18th centuries is overwhelmingly the history of answers to this question” (Haakonssen, 1985, p. 241).
Understanding Aquinas’ Eternal Law: The Divine Blueprint
To better understand what Grotius was reacting to, we must consider the views of Thomas Aquinas. In Aquinas’s philosophy, eternal law refers to the divine order established by God, which governs the entire universe. It is the perfect, unchanging blueprint in the mind of God. Natural law, then, is the human participation in this eternal law through reason.
Aquinas writes: “… the natural law is nothing else than the rational creature’s participation of the eternal law” (ST I–II, Q.91, Art.2).
He emphasizes that law must be oriented toward the common good: “Consequently the law must needs regard principally the relationship to happiness. Moreover, since every part is ordained to the whole, as imperfect to perfect; and since one man is a part of the perfect community, the law must needs regard properly the relationship to universal happiness” (ST I–II, Q.90, Art.2).
Here, happiness (beatitudo) doesn’t mean personal pleasure or success but rather ultimate fulfillment in God—the highest good toward which all human beings naturally strive. This fulfillment arises through social life. In the Aristotelian-Thomistic tradition, one cannot fully understand the individual apart from their role within the community—family, society, and the political order.
This stands in sharp contrast to modern thinkers like Hobbes and Locke, who begin with the image of the isolated individual—autonomous, self-interested, and prior to society. In their frameworks, natural law becomes a mechanism for securing individual rights. For Aquinas, by contrast, law is a moral guide directing the community toward the common good.
H. Kainz elaborates: “Natural law thus is actual participation in the eternal law” (Kainz, 2004, p. 19). This view anchors morality in a cosmic, theological framework. God, as the eternal legislator, imprints law onto the human soul; human beings respond to this divine impression through their natural inclinations.
Grotius, however, made a groundbreaking shift. As Kainz observes: “The idea that natural law might be valid and binding even if God did not exist had been suggested before Grotius, but … Grotius made this point more explicitly and forcibly. This was a groundbreaking proto-modern attempt to disengage natural law from the question of the existence of Divine Legislator” (Kainz, 2004, p. 20).
Grotius argued that natural law would still be valid—even without God—because it is based on the rational and social nature of human beings. This move laid the foundation for modern secular theories of rights and law.
Understanding this transition—from Aquinas’s divine cosmic order to Grotius’s rational secularism—is key to understanding the development of modern moral and political thought. This shift shaped the philosophies of Hobbes, Locke, and many others who followed, grounding the idea of law and rights not in divine will but in human nature itself.
De Jure Belli ac Pacis
Grotius articulated a revolutionary understanding of natural law that placed reason—and not solely divine command—at its foundation. As he writes in the Prolegomena to De Jure Belli ac Pacis:
“Natural right is the dictate of right reason, indicating the moral turpitude or moral necessity of any act from its agreement or disagreement with a rational nature, and consequently that such an act is either forbidden or commanded by God, the author of nature.”(Grotius, Hugo, Prolegomena, §8)
“Now the Law of Nature is so unalterable, that it cannot be changed even by God himself. For although the power of God is infinite, yet there are some things, to which it does not extend. Because the things so expressed would have no true meaning, but imply a contradiction. Thus two and two must make four, nor is it possible to be otherwise; nor, again, can what is really evil not be evil.”(Grotius, Hugo, Book I, Chapter 1, §10)
De Jure Belli ac Pacis and the Theory of the Social Contract
Civil right derives from the civil power. The civil power is the sovereign power of the state. A state is a perfect body of free men, united together in order to enjoy common rights and advantages.
“There may be many reasons indeed why a people may entirely relinquish their rights, and surrender them to another: for instance, they may have no other means of securing themselves from the danger of immediate destruction; or, under the pressure of famine, it may be the only way through which they can procure support.”
(Hugo Grotius, De Jure Belli ac Pacis, Book I, Chapter 3)
Grotius acknowledged that political authority could arise under duress—through necessity, famine, or imminent threat—and not always through a voluntary social contract. In such cases, people might surrender their rights not out of free will but as a matter of survival. He also recognized that rulers could emerge not only by agreement but by conquest and power. Importantly, legitimacy did not always rest on whether the ruler governed for the people’s good, but rather on whether authority could ensure order and stability.
Grotius and the Rise of Contractarianism
While Grotius is often classified as a contractarian, his position differs significantly from Hobbes or Locke. He does use the concept of a social contract to explain the emergence of government, but for Grotius, natural law is not based on any human agreement or divine will. Instead, it exists independently—famously stating that natural law would remain valid “even if God did not exist” (etiamsi daremus non esse Deum).
In Grotius’s view, political society arises not from fear or chaos—as Hobbes would later argue—but from human beings’ rational and social nature (societas). People recognize the necessity of laws and authority in order to secure peace and justice. For Grotius, the contract forms the government, not society itself. He believes that even in the absence of formal authority, human sociability can maintain a basic level of order.
Several key ideas follow from this. Human beings possess both passions and reason, but it is reason that allows them to discern natural law and act according to it. Grotius writes that every species uses its best faculties to survive, and for humans, that faculty is reason. “Now right reason and the nature of society—which claims the second, and indeed more important place in this inquiry—prohibit not all force, but only that which is repugnant to society, by depriving another of his right.” (De Jure Belli ac Pacis, Book I, Chapter 2)
Grotius is relatively optimistic: he believes that humanity’s natural sociability (societas) is enough to sustain community and ensure a just life. Of course, critics contest this view—Hobbes, for example, strongly doubted natural sociability and stressed the necessity of absolute authority to prevent conflict.Yet this debate between reasoned sociability and coercive authority would shape much of the political philosophy that followed in the 17th and 18th centuries.
Bibliography
- Haakonssen, Knud. “Hugo Grotius on the History of Political Thought.” Political Theory 13, no. 2 (1985): 239–265.
- Kainz, Howard P. Natural Law: An Introduction and Re-examination. Chicago: Open Court, 2004.
- Tuck, Richard. Natural Law Theories. Cambridge: Cambridge University Press, 2002.
- Moore, James. “Calvinists, Arminians, Socinians: Popular Sovereignty and Natural Rights in Early Modern Political Thought.” In Philosophy, Rights and Natural Law: Essays in Honour of Knud Haakonssen, edited by Ian Hunter and David B. Saunders, 107–130. Edinburgh: Edinburgh University Press, 2019.
- Grotius, Hugo. On the Sovereignty of the Supreme Power in Sacred Affairs (1614). Translated by Francis Oakley. New Haven: Yale University Press, forthcoming/available online editions may vary.
- Grotius, Hugo. De Jure Belli ac Pacis (On the Law of War and Peace). 1625. Translated by Francis W. Kelsey. Oxford: Clarendon Press, 1925. (Reprints available.)
- Finnis, John. Natural Law and Natural Rights. 2nd ed. Oxford: Oxford University Press, 2011.
Portrait of Hugo Grotius (circa 1631) by Michiel Jansz. van Mierevelt. Source: Wikimedia Commons, Public Domain — free for use.
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