Leibniz on Slavery and the Ownership of Human Beings

1. Introduction

Around 1703, Leibniz composes an untitled French text that editors have chosen to call “Sur la notion commune de la justice,” that is, “On the Common Notion of Justice.”¹ This text contains an argument against the moral permissibility of chattel slavery, or slavery understood as the literal ownership of a person. My goal in this paper is to analyze and evaluate this argument. Leibniz’s discussion of slavery in this text is just over two pages long. Yet, it raises almost more questions than it answers, and these questions have not been addressed in the secondary literature so far.² The biggest question is how exactly Leibniz’s argument is supposed to work—or rather, his arguments, because there appear to be several. The relevant paragraphs are so densely written and contain so much legal terminology that it is quite difficult to untangle, interpret, and evaluate the arguments. Another question is what exactly Leibniz’s position on slavery is. For instance, does he hold that slavery in general is impermissible, or only its abusive forms? Leibniz grants certain things for the sake of argument, which makes it hard to figure out just how robust his opposition to slavery turns out to be. A final and related question concerns the theological doctrines that Leibniz invokes, such as the immortality of the soul and the special status that human beings have in God’s plan: how important are these doctrines for the argument? Does Leibniz also offer independent reasons against the permissibility of slavery that could persuade someone who rejects these theological claims? My paper aims to answer these questions, which requires a careful analysis of the pertinent passages from “On the Common Notion of Justice” and their context. The relevant context includes not just the rest of this text, but also other writings by Leibniz as well as legal and philosophical texts by other authors that serve as important background.

On the interpretation I will defend, we learn a number of important things from “On the Common Notion of Justice” about Leibniz’s views on the moral status of slavery as well as on moral rights and obligations more generally. First, he rejects the Aristotelian claim that some human beings are ‘natural slaves,’ that is, that they lack the capacity for self-government and may hence legitimately be enslaved. Instead, Leibniz argues that all human beings have a special moral status, partly due to their place in God’s plan and partly due to their capacity for rationality—a capacity that we must assume even in cases in which there are no clear signs of rationality. Among other things, human beings have inalienable rights to freedom, control over their own bodies, and membership in the moral community of which God is the sovereign. As a result, humans possess a range of moral obligations toward each other: they must refrain from harming others, they must treat others equitably, and they must even pursue the happiness and virtue of others as well as the perfection of the moral community as a whole. These moral rights and obligations are incompatible with chattel slavery because they make absolute ownership rights over human beings morally illegitimate. Indeed, Leibniz’s ethics requires that slaveholders pursue the happiness and virtue of slaves, which is arguably incompatible even with forms of slavery that do not give slaveholders absolute ownership rights.

When Leibniz composed this text, there was an ongoing debate about slavery in Europe, prompted partially by the growth of the Atlantic slave trade and partially by theoretical concerns arising in various ethical theories, particularly in the natural law tradition. Influential natural law theorists like Francisco Suárez, Hugo Grotius, Samuel von Pufendorf, Thomas Hobbes, Christian Thomasius, and John Locke wondered, for instance, whether the master-slave relationship is a natural relationship and if it is not, whether it can arise in morally justifiable ways. Because this tradition typically views human beings as naturally free³ and nevertheless accepts some forms of slavery, these philosophers endorsed various explanations of how slavery can legitimately arise. Two such explanations were quite uncontroversial: (i) a person can voluntarily become a slave, that is, subject herself to a master in exchange for food and shelter, and (ii) a person captured in a just war can be enslaved. In both cases, natural law theorists typically argued that this person has forfeited their natural freedom, at least to some extent.⁴ Some viewed penal slavery as a third legitimate form of slavery.⁵ What was more controversial is whether there are other morally permissible ways of becoming a slave, for instance by being born the child of slaves.⁶ Likewise, it was controversial just how extensive the rights of slaveholders over their slaves are. Some philosophers, such as Hobbes and Locke, held that at least in the case of slaves who were captured in a just war, the slaveholder has absolute rights.⁷ Others argued for much more limited rights.⁸

Leibniz must have been familiar with at least portions of this debate; in fact, his own argument appears to borrow from the natural law tradition when he talks about our natural right to freedom. Of course, he is not the first early modern author to raise objections to the doctrine that slaves are literally the property of slaveholders⁹ or to the Aristotelian doctrine that some human beings are natural slaves.¹⁰ Nevertheless, what I just said about the ongoing debate shows that Leibniz’s claims about slavery were quite controversial at the time; he deserves credit for saying these things, even though he could have made his points much more explicitly—not to mention more publicly, since Leibniz did not publish “On the Common Notion of Justice.”¹¹

The first step of my analysis is the careful disentanglement of three different layers, or sub-arguments, of Leibniz’s argument against slavery in “On the Common Notion of Justice.” One layer is Leibniz’s contention that slavery, construed as the ownership of human beings, is not in accordance with the nature of things. A second layer consists of what Leibniz says while granting, for the sake of argument, that slavery is in accordance with the nature of things. On that assumption, he argues, the rights of the slaveholder would still be severely limited by other moral obligations—or, more specifically, by the demands of equity, charity, and piety. A third and final layer is Leibniz’s discussion of, and partial agreement with, Aristotle’s doctrine of natural slaves. This layer is difficult to interpret because it is initially quite unclear—at least without consulting other texts—to what extent Leibniz agrees with Aristotle.

The structure of this paper is as follows. In section 2, I consider the context of Leibniz’s discussion of slavery in “On the Common Notion of Justice” and then separate the three layers of his argument. In the subsequent three sections, I explore these layers more thoroughly, one by one. In the final and sixth section, I briefly evaluate Leibniz’s argument as a whole.

One important caveat is that this paper will not directly address race, even though there are important connections between slavery and race. Indeed, during Leibniz’s lifetime, slavery came to be tied to race much more than before. Unfortunately, my paper cannot discuss this important issue and instead focuses on whether it is ever morally permissible, according to Leibniz, for any human being to own any other human being. After all, I focus on Leibniz’s arguments against slavery from “On the Common Notion of Justice,” in which race does not play an explicit role. For more on the role of race in Leibniz’s system, see Fenves (2005), Smith (2015), and Harfouch (2017).

A related caveat is that this paper is an attempt to interpret the arguments that Leibniz provides in “On the Common Notion of Justice,” rather than a comprehensive examination of his views on slavery. The latter is a much more complicated issue that cannot be covered adequately in a journal article. One complication is that in legal texts, Leibniz simply appears to accept slavery as a legal category, without saying anything explicit about its moral status. For instance, he sometimes notes in his early legal writings that according to the law (that is, Roman law), slaves are considered to be things [res] rather than persons (e.g., Nova Methodus Discendae Docendaeque Jurisprudentiae 2.15, A 6.1.301).¹² Yet, it is unclear in those texts whether Leibniz holds that as a result of this legal status, slaves lack the moral status of persons. It is also unclear in those texts whether Leibniz takes the legal status of slaves to be morally justifiable. Indeed, “On the Common Notion of Justice” is the only text, to the best of my knowledge, in which Leibniz explicitly addresses the moral status of slavery.¹³ Thus, I will only draw on other texts insofar as they shed light on what Leibniz might mean in this text.

As a result of the second caveat, I will not be able to discuss a noteworthy and rather disturbing early text in which Leibniz advocates abducting boys from Africa and other places in order to turn them into warrior slaves (Modus instituendi militam novam invictam, 1671–72, A 4.1.408–10). This text discusses neither the legal nor the moral status of slavery; it is a policy proposal with political and practical rather than theoretical aims. Of course, it is perfectly legitimate to wonder why Leibniz would make such a proposal unless he views slavery as legally and morally permissible. Thus, this text may give us reasons to classify the early Leibniz as a proponent of slavery. If this is the case, he appears to have changed his mind about slavery by the time he composes “On the Common Notion of Justice” thirty years later. Unfortunately, analyzing the Modus instituendi and its relationship to “On the Common Notion of Justice” would take us too far afield. For helpful discussions of Leibniz’s proposal about warrior slaves, see Harfouch (2017), Smith (2015: 160ff.), Fenves (2005), and Cook (forthcoming).

2. The Argument’s Context and Its Three Layers

There are a few features of “On the Common Notion of Justice” that make it hard to interpret the discussion of slavery it contains. One problem is that Leibniz never finished this text; it breaks off mid-sentence, just one paragraph after the discussion of slavery. It appears that Leibniz intended to compose at least another paragraph—possibly summing up what he took to be the most important result of his discussion. Having that paragraph might have been a significant help in interpreting the text. Even though Leibniz edited this text quite a bit—there are many corrections and additions in the sole extant manuscript (which is in Leibniz’s hand)—it was clearly not entirely finished.¹⁴

Another difficulty is the somewhat idiosyncratic context in which the discussion of slavery arises. As we will see, this context explains why Leibniz focuses on certain aspects of slavery and why he frames his argument in the way that he does. Thus, before separating the three different layers of the discussion, let us examine this context.

Leibniz’s aim in “On the Common Notion of Justice” is to analyze the concept ‘justice.’ His preferred definition of justice is goodness conformed to wisdom (NCJ 172/Riley 59), or the charity of the wise, that is, goodness toward everybody else that conforms to wisdom.¹⁵ In the course of arguing for this definition, Leibniz distinguishes three degrees of right or justice, as he did ten years earlier in the preface to the Codex Juris Gentium Diplomaticus (A 4.5.61f./L 422f.) and decades before that in the Nova Methodus Discendae Docendaeque Jurisprudentiae (2.73ff., A 6.1.343f.). One of the main tasks of “On the Common Notion of Justice” is to argue that it is rational, and indeed morally obligatory, to be just in all three ways.

The first and least demanding degree of right is what Leibniz calls ius strictum, that is, strict (or rigorous) law or right,¹⁶ which requires only that we do not harm others. The second and far more demanding degree is equity, which goes beyond ius strictum because it demands that we follow the Golden Rule. The principle of equity, as Leibniz argues in “On the Common Notion of Justice,” requires for instance that we help others who are in need when we can easily do so. Sometimes Leibniz distinguishes equity from charity in a narrow sense, which requires that we work for the happiness of others (NCJ 177/Riley 63). Equity and this narrow sense of charity jointly constitute the second degree of right.¹⁷ The third and most demanding degree of right is piety, which requires that we live virtuously, in imitation of God. Piety demands not just that we treat others in accordance with the principles of equity and narrow charity, but also, among other things, that we are virtuous in actions that do not concern other people. More generally, Leibniz sometimes characterizes piety as containing all the virtues and as being based on a ‘consideration of God or a government which imitates that of God;’ it requires us to pursue the general good (NCJ 173/Riley 60; see also the preface to the Codex Juris Gentium Diplomaticus A 4.5.62f./L 423). Piety also provides us with additional motivation for acting virtuously through special rewards and punishments, as well as through the desire to conform our will to the divine will. In describing these three degrees, Leibniz uses Latin slogans from the Justinian Institutes, which enumerate three legal precepts: ‘to live honestly, not to injure another, and to give to each one that which belongs to him’ (Institutes 1.3).¹⁸ These three precepts correspond to the three degrees of right: the ius strictum requires that we do not injure another, equity requires that we give others what belongs to them, and piety requires that we live honorably.¹⁹

Toward the end of “On the Common Notion of Justice,” Leibniz discusses the Hobbesian state of nature, that is, a state in which there is no government. Hobbes claims that in such a state, there are no property rights; every person would have an equal right to all things (Leviathan 13.13, 78 and De cive 6.1). Leibniz disagrees with this and insists that even in a state of nature there would be property. He observes that property rights can ordinarily be acquired in at least three ways: by producing something new, by taking possession of something that nobody else has possessed and making improvements to it, and by acquiring something from its rightful owner (NCJ 175/Riley 61f.). Then Leibniz notes that this raises the question of whether parents are the rightful owners of their children; they do, after all, produce these children and improve them through education. It also raises the question of whether slaveholders are the rightful owners of the children of their slaves (NCJ 175f./Riley 62). As Leibniz points out, Robert Filmer, the English political theorist and famous proponent of the divine right of kings, answers both of these questions affirmatively.²⁰ Yet, Leibniz claims that Filmer is taking ownership rights too far: even though parents possess an extensive power over their children, this does not mean that they literally own them, as one owns a horse or a dog that one has bred (NCJ 176/Riley 62).

Leibniz then turns to a longer discussion of slavery in response to a potential objection. This objection is that since slaves are, by the law of nations,²¹ the property of their masters, as are the children of slaves, ‘no reason can be seen why the children whom we have produced and formed by education are not our slaves by an even juster title than those whom we have bought or captured’ (NCJ 176/Riley 62). In other words, the objection is that since the literal ownership of slaves is accepted by all nations, there is no good reason to deny the ownership of children by their parents.²² After all, parents have an even stronger claim to ownership of their children than a slaveholder has to ownership of his slaves and their children.

This context is important for understanding the ensuing discussion of slavery. We need to keep in mind that Leibniz’s primary goal in the three paragraphs that contain his argument against slavery is to respond to the claim that children are the property of their parents for the same reason that slaves and the children of slaves are the property of the slaveholder. In other words, the text is not first and foremost a discussion of the permissibility slavery.²³ Leibniz aims to investigate primarily whether parents have an absolute right—that is, full ownership rights—over their children and only secondarily whether slaveholders have an absolute right over slaves. As a result, we should not expect Leibniz to explore the moral status of slavery systematically or comprehensively; it makes sense for him to focus on the most severe form of slavery, which gives slaveholders unlimited rights over slaves.²⁴ This goal also explains why Leibniz assumes during parts of his discussion, for the sake of argument, that the ownership of slaves is in accordance with natural law; he thinks that some responses to the objection work even when granting that slaves are property.

Leibniz puts forward several replies to the potential objection. These replies can be separated into three sub-arguments or layers, corresponding to the three paragraphs of Leibniz’s discussion. Leibniz starts by saying that ‘even if one were to grant that there is a right of slavery among men which conforms to natural reason’ (NCJ 176/Riley 62, translation altered), this right would have severe limitations; it would not be an absolute right. After all, there are other natural rights that are stronger than this supposed right of slaveholders over their slaves and that will, at the very least, prevent slaveholders from ‘making a slave vicious or unhappy’ (NCJ 177/Riley 63, translation altered). I will say more about these other rights below. What is important for present purposes is simply that according to Leibniz, slaveholders cannot possess unlimited rights over slaves because that would be a violation of other, stronger rights. Slavery, viewed as the literal or complete ownership of another person, is impermissible. This is the first layer of Leibniz’s discussion of slavery.

In the next paragraph, Leibniz provides a distinct response to the potential objection, which he introduces as follows: ‘if I were to grant, contrary to the nature of things, that a man is the property of another man …’ (NCJ 177/Riley 63; translation altered). Thus, Leibniz is here making a point that works even when granting, for the sake of argument, that slaves are the property of slaveholders. The point he makes is, roughly, that the principles of equity, charity, and piety would impose extensive obligations on slaveholders, even if slaveholding were generally permissible. This is the second layer of Leibniz’s argument.

In the subsequent paragraph, Leibniz applies this reasoning to the case of children and argues that it is even clearer in that case that there is no absolute ownership. Here, Leibniz invokes Aristotle in support, even though it is evident that Leibniz agrees with Aristotle only partially. He introduces one final consideration to adjudicate the question of whether the ownership of persons is morally permissible. This final consideration, which is connected in various ways to the considerations mentioned earlier, is whether such ownership is in accordance with the general order of things, that is, with what is overall most fitting. Leibniz appears to agree with Aristotle that the ownership of another person would be fitting only in cases in which this other person is incapable of conducting herself well and therefore deserves to be someone else’s property. This is a reference to Aristotle’s theory that there are ‘natural slaves,’ as we will see in section 5. While it is not entirely clear what Leibniz concludes from this consideration, it seems that he ultimately holds that this justifies neither the ownership of children nor of slaves, because both groups have the capacity to learn how to conduct themselves well. This is the third layer.

In the next three sections, I will examine each of the layers in turn.

3. Layer 1: Slavery as Against the Nature of Things

As mentioned in the previous section, the first layer of Leibniz’s discussion of slavery is the argument that the rights of a slaveholder over slaves cannot be absolute or unlimited rights. After all, even if slaveholders had rights over slaves in accordance with the ius strictum,²⁵ there are other, stronger rights that oppose—or at least severely limit—these former rights. This, in my view, is the most important layer of the argument, and I will spend more time on it than on the other two layers. Leibniz starts spelling out these slavery-opposing rights as follows:

This is the right of rational souls which are naturally and inalienably free; it is the right of God, who is the sovereign master of bodies and of souls, and under whom masters are the fellow-citizens of their slaves, since the latter have the right of citizenship in the kingdom of God as well as their masters. (NCJ 177/Riley 62)

Leibniz here mentions three rights: (a) the right of rational souls to be free, (b) the right of God to govern his creatures, and (c) the right of rational souls to be citizens in God’s kingdom.²⁶

Let us start with the first right, which is particularly important for understanding Leibniz’s relation to the natural law tradition. At first glance, this right may seem to conflict with the second right: how can rational souls be ‘naturally and inalienably free’ if God has the right to be the sovereign master of these souls? Doesn’t the second right entail that human beings are God’s slaves (or at least subjects), and hence not entirely free? Or, to put it differently, how can the first right make it impermissible for a human being to be the master of another human being, without also ruling out divine mastery? The answer is presumably that God’s mastery differs from the mastery of a human slaveholder in such a way that only the latter, but not the former, undermines freedom. This makes sense because for Leibniz, the aim of God’s rule is the happiness of all rational souls; he wants all of his rational creatures to be as virtuous and happy as possible and has ordered the world in the way that is most conducive to their virtue and happiness (letter to Arnauld, October 19, 1687, A 2.2.257f./LAr 263; Theodicy §118). Leibniz plausibly holds that this kind of loving rule does not constrain the freedom of the subjects; in fact, this type of rule might increase their freedom by helping them to become more virtuous and rational, and making true happiness possible. Moreover, Leibniz claims in “On the Common Notion of Justice” that once human beings know God, they will love God and thus obey him voluntarily, with the greatest pleasure (NCJ 172/Riley 59). Thus, God’s rule is compatible with human freedom because as soon as humans know God, they will freely choose to do what he wants them to do.²⁷

Another important question about the right of rational souls to be free is why Leibniz might think that all rational souls have this right. He does not explicitly justify this claim in “On the Common Notion of Justice.” In his early work Nova Methodus, he says merely that nature is the cause of liberty (2.17, A 6.1.303). Yet, as already seen in the brief discussion of the natural law tradition, the doctrine that human beings are naturally free was relatively common, though by no means uncontroversial, in the early modern period. Filmer, whom Leibniz targets in his discussion of slavery, is the author of an entire treatise arguing against what he identifies as the ‘common opinion’ that ‘Mankind is naturally endowed and born with freedom from all subjection,’ or that there is a ‘natural liberty and equality of mankind’ (Patriarcha I.1, Sommerville 2f.).²⁸ John Locke, in turn, writes his Two Treatises of Government to argue against Filmer and in favor of the natural liberty and equality of all men; he claims, for instance, that the natural state of men is ‘a State of perfect Freedom’ (Two Treatises 2.4; see also 2.54). For Locke, this freedom that each man possesses by nature is ‘grounded on his having Reason’ (Two Treatises 2.63).²⁹

Thus, the claim that all humans are born free was a widely accepted claim in Leibniz’s day, particularly among natural law theorists. It does not originate in the early modern period, however. It goes back to antiquity and is quite commonplace in Roman writings, arguably as a result of Stoic influences.³⁰ The early third century Syrian jurist Ulpian, for instance, writes that ‘everyone would be born free by the natural law,’ even though the ius gentium permits slavery (Digest 1.1.4). Similarly, the late fourth century Roman politician Florentinus defines slavery as ‘an institution of the jus gentium, whereby someone is against nature made subject to the ownership of another’ (Digest 1.5.4.1). Thus, these Roman writers acknowledge, just like Leibniz and later natural law theorists, that slavery is in an important sense unnatural because every human being is by nature born free.

There is, however, a crucial difference between Leibniz and some other writers who claim that slavery conflicts with our natural freedom. For at least some of these other writers, saying that humans are by nature free, or that slavery is against nature, merely means that humans are free when they are born or when in the state of nature. This becomes clear, for instance, in the late seventeenth century book De jure civitatis by the Dutch jurist and philosopher Ulrik Huber: after citing Florentinus’s definition of slavery as an institution that is against nature, Huber claims that ‘against nature’ means merely ‘against the state of nature, or the youthful condition of men, not against natural law [jus Naturæ], or the dictate of right reason’ (2.1.6.6, 1698: 334).³¹ Thus, saying that human beings are naturally free need not imply that slavery is morally impermissible, as we already saw earlier when discussing justifications of slavery in the natural law tradition. Natural freedom can after all be viewed as alienable, that is, there might be legitimate ways to become unfree. In contrast, Leibniz clearly holds that slavery violates natural law and is morally impermissible.³² After all, he says in the passage under discussion that our natural right to freedom is inalienable, and that this right is opposed to, and stronger than, any alleged right to own slaves. Leibniz thus has a far more robust understanding of the natural freedom of human beings than many of his contemporaries.

Let us now turn to the second right, that is, God’s right to govern his creatures. By bringing it up in this context, Leibniz appears to be suggesting that God’s right over his creatures directly entails that no human being can have full ownership rights over another human being. The thought might be that if God is our rightful sovereign ruler or master, we cannot additionally have a human master who has absolute rights over us. Two persons may co-own or co-rule something, but they cannot both have absolute rights. Hobbes says this explicitly in De cive: ‘Dominion, i.e. sovereign power, is indivisible, so that no one can serve two masters’ (9.1). This also makes sense independently.

It is tempting to think that God has the right to govern his creatures either in virtue of his omnipotence, or because he created us. In fact, the latter would fit well with what Leibniz says earlier in this text about the ways of acquiring property: one of these ways, as already seen, is to produce something new. Yet, this cannot be quite right; after all, Leibniz is in the process of arguing against the doctrine that since parents produce their children, these children are the property of their parents. Moreover, Leibniz says explicitly in other places that he does not agree with the Hobbesian claim that being all-powerful makes God the rightful ruler of human beings. Power in itself is not good and does not justify ruling the less powerful; otherwise, powerful tyrants, despots, and the devil would be justified in ruling human beings (RH 12; NBJ 147–51/Riley 47f. and NBJ 155/Riley 50). Hence, there must be another justification for God’s right to govern his creatures. But what is this justification?

The answer appears to be that God’s perfect goodness justifies his status as the rightful sovereign of all rational souls. After all, in “On the Common Notion of Justice,” Leibniz says that our knowledge of God’s power, wisdom, and goodness ‘should make us envisage God as the sovereign monarch of the universe whose government is the most perfect State that one can conceive, where nothing is neglected, where every hair on our head is counted, where all right becomes fact’ (NCJ 171/Riley 58). In other words, we should think of God as the sovereign precisely because he is wise and good, not only because he is powerful. In a letter to Thomas Burnett from February 13, 1700, Leibniz even says explicitly that ‘government naturally belongs to the best’ (A 1.18.388; see also 380). In addition to this general justification for God’s government of all creatures, it is important to note that rational souls (or minds) have a special status in the created world: they resemble God and form what Leibniz calls a society of minds or City of God, which is a moral kingdom within the natural world.³³ Their relation to God is very different from that of animals.³⁴ Because God is the most perfect mind and aims for the perfection of created minds, he is their natural sovereign. This is completely unlike God’s relation to non-rational creatures, whom God governs merely as an architect or engineer.³⁵

Corresponding to God’s right to govern his rational creatures, there is also a right of rational creatures to be members of God’s moral kingdom, which I labeled (c) earlier. Leibniz appears to hold that every creature that is capable of morality—that is, every rational creature—has a right to be treated justly, even by God, and therefore a right to be included in God’s moral kingdom.³⁶ This right appears relevant for slavery in at least two ways. First, it might entail a type of equality that is incompatible with chattel slavery. Leibniz says, after all, that slaveholders are ‘the fellow-citizens of their slaves’ in this kingdom and that slaves and slaveholders have an equal right of citizenship (NCJ 177/Riley 62). The idea might be that in this kingdom, all humans have the same rights and are subordinated, in the relevant sense, only to God. No human being can be completely subordinated to another. Second, being a member of this kingdom means being subject to perfect justice.³⁷ Thus, every member must obey the moral principles that we will explore in section 4; these principles, as we will see later, at the very least put severe restrictions on the rights of slaveholders and are arguably incompatible with many, if not all, forms of slavery.

Immediately after listing the three rights, Leibniz explains why human beings cannot be owned by other human beings in the following way: ‘One can say, then, that the body of a man belongs to his soul, and cannot be taken away from him while he is living; now, the soul not being able to be acquired, the ownership of the body cannot be acquired either’ (NCJ 177/Riley 62, translation altered). Here, Leibniz appears to advance the following argument:

1. The body of a man can only be the property of his soul.

2. The soul of a man cannot be acquired.

3. If x can only be the property of y, and y cannot be acquired, then x cannot be acquired. [implicit] Therefore, the body of a man cannot be acquired.

Let us examine how persuasive this argument is. The first premise is comparatively uncontroversial: on any standard conception of the soul, it is true that while a person is alive, her body is subordinated to, and directly controlled by, her soul.³⁸ It makes sense to understand this as implying that the soul possesses, and maybe even owns, its body.³⁹ A human being cannot take possession of another human being’s body in the way that we can take possession of another human being’s house or money; you cannot take their body away from them, while they are alive. Thus, the only way to own another human being is to own their soul along with their body.⁴⁰ The third premise also seems very plausible. But why would someone who wants to defend the right to slavery agree with the second premise, which states that souls cannot be acquired? To defend this claim, Leibniz could say at least three things. First, and most promisingly, he could again invoke the special status of rational souls: they are naturally and inalienably free.⁴¹ Second, he could say that it is simply impossible to take ownership of someone else’s soul, at least against that person’s will. As he states in a marginal note to Definitionum Juris Specimen, if you capture someone in a war, that person may be in your possession, but you do not thereby acquire that person, unless they surrender (A 6.3.608f.). In other words, a person would have to subordinate themselves to someone else voluntarily in order to be acquired or owned by this other person.⁴² This is of course not quite strong enough to justify the second premise, because it allows for souls to be acquired when the person surrenders voluntarily. Yet, it is strong enough to rule out many forms of slavery. Third, Leibniz could invoke his theory of monadic domination: created rational souls dominate an organic body throughout their existence, and these souls cannot themselves come to be dominated by any other monad.⁴³ Of course, this third approach depends on Leibniz’s idiosyncratic metaphysics and might therefore not be very effective rhetorically.

Leibniz spells out the consequences of this argument’s conclusion for the institution of slavery as follows: ‘Thus, the right of the master over the slave can only be like what is called a servitude on the land of another,⁴⁴ or like a species of usufruct; but usufruct has its limits: it must be exercised salva re so that this right cannot go to the point of making a slave vicious or unhappy’ (NCJ 177/Riley 62f., translation altered). Leibniz is using a lot of legal terminology here, which needs to be clarified before proceeding. Servitude with respect to land means that one person has the right to use another person’s land in specific ways; for instance, the former may have the right to let her animals graze on the other person’s land, or to use a well or road on that land (Shumaker and Longsdorf 1912: 842; Colognesi 2016: 529). Such a servitude is very different from ownership: one is allowed to use the land for certain circumscribed purposes, but one does not have full ownership rights. For instance, one cannot sell that land. As a result, this type of servitude is closely connected to the notion of ‘usufruct,’ which is a right of using and enjoying the fruits of a certain object (for instance of a piece of land) without having full ownership rights.⁴⁵ Because holders of usufruct do not fully own the object, there are strict limits to what they can do with it. For example, usufruct rights might entitle them to cultivate a piece of land or live in a certain house, but not to substantially alter this land or house (see Colognesi 2016: 529). This already explains the meaning of salva re: a holder of usufruct must not alter or impair the object itself, that is, its substance (see Digest 7.1.1).⁴⁶

Given the meanings of these legal notions, Leibniz’s basic point seems to be the following. Because the only rightful owner of a person’s body is that person’s soul, another person can at most have a right of usufruct over the former person’s body, that is, they can at most have the right to the fruits of that person’s labor.⁴⁷ Since this does not rise to the level of ownership, it comes with severe constraints. The holder of such a right is not allowed to kill or permanently alter the body of the other human being; they are not even allowed to make that person vicious or unhappy, according to Leibniz. The idea here seems to be that making the person vicious or unhappy would mean that one is not using them salva re, that is, that one is impairing or ruining that person, which is not included in the rights of usufruct. Thus, slavery understood as literal ownership is ruled out, as is any type of domination that deprives its subject of virtue or happiness.

Before concluding the discussion of the first layer, it is important to note that Leibniz frames the point that there is a stronger right countering the supposed right of slavery—or rather, three stronger rights, since they appear to be distinct—somewhat unclearly. First of all, he says that the stronger rights oppose ‘the abuse of this right [of slavery]’ (NCJ 176/Riley 62). This may make it sound as if only the abuse of the right of slavery is a problem, not slavery more generally. In other words, it may seem as if Leibniz’s criticism of the institution of slavery is not very robust at all. Moreover, Leibniz starts the paragraph by saying, ‘… even if one were to grant that there is a right of slavery among men which conforms to natural reason…’ (NCJ 176/Riley 62, translation altered). He does not say explicitly whether he believes that there is a right of slavery that conforms to natural reason.⁴⁸ Yet, the fact that the verb is in the subjunctive mood suggests that Leibniz does not believe there is such a right. Moreover, there are other hints that this paragraph is supposed to show that a right of slavery does not, in fact, conform to natural reason; this also suggests that slavery itself (when understood as chattel slavery, or the literal ownership of another person) is ruled out, and not only its abusive forms. One hint is that the subsequent paragraph begins as follows: ‘But if I were to grant, contrary to the nature of things, that a man is the property of another man…’ (NCJ 177/Riley 63, translation altered). This indicates that Leibniz takes himself to have shown in the paragraph we are currently discussing that slavery is contrary to the nature of things. A final piece of evidence is internal to the paragraph under discussion. If men are by nature born with an inalienable freedom and if it is impossible to acquire the soul of another person, then it seems that natural reason rules out chattel slavery. Thus, the argument is quite robust: it establishes that chattel slavery in general—not just its abusive forms—is against the nature of things and a violation of natural rights.

4. Layer 2: Limitations on the Rights of a Slaveholder

The second layer of Leibniz’s argument is much more straightforward than the first or third. Leibniz’s point in the paragraph that corresponds to this second layer is quite clear: even if an absolute right of slaveholders over slaves were not against nature, it would still violate the laws of equity, charity, and piety (NCJ 177/Riley 63). Recall that Leibniz acknowledges three degrees of right or justice; ius strictum, which among other things governs property rights,⁴⁹ is the least demanding of the three degrees. It requires only that we do no harm, which presumably includes harm to persons and to their property. The next degree, equity, goes beyond the first degree in demanding that we follow the principle of equity, which Leibniz sometimes identifies with the Golden Rule: ‘one should grant [to others] whatever one would wish in a similar situation, without claiming to be privileged, against reason, or [without claiming] to be able to allege one’s will as a reason’ (NCJ 167/Riley 56). According to Leibniz, this principle requires both that we help those in need when we can easily do so, and what he sometimes calls charity in the narrow sense: we must pursue the good or happiness of others when it is not too costly for us, because we would complain if someone refused to do the same for us (NCJ 167/Riley 55). The third and most demanding degree of right is piety, which requires that we live virtuously more generally and imitate God in aiming at the general good (NCJ 173/Riley 60; Codex Juris Gentium Diplomaticus A 4.5.62f./L 423).

According to Leibniz, someone who believes in God and the immortality of the soul has stronger reasons to act equitably and charitably than someone who does not. After all, a theist—at least a Leibnizian theist—believes that there are divine rewards for all good deeds and punishments for all bad deeds (NCJ 171f./Riley 58f.). Moreover, Leibniz argues that whoever knows God will desire to conform their will to God’s will; they will want to act justly not only out of hope for rewards and fear punishment, but also out of love for God (NCJ 172/Riley 59). This also allows believers to act in accordance with the highest degree of justice, that is, piety.

Yet, importantly, Leibniz also provides justifications for acting in accordance with the ius strictum and with the principles of equity and charity that are independent of theological doctrines and have force even for someone who does not believe in God. Thus, acting in accordance with the first two degrees of right is morally obligatory even for atheists. Ultimately, the normativity of these principles derives from the desire for one’s own happiness that every human being possesses.⁵⁰ As Leibniz puts it in “Principles of Pufendorf” in 1706, ‘there would be a natural obligation even on the hypothesis … that God does not exist, … since care for one’s own preservation and well-being certainly lays on men many requirements about taking care of others’ (Dutens 4.3.279f./Riley 71). In other words, the atheist’s obligation to help others is grounded in her concern for her own happiness and self-preservation. This also becomes clear in the preface to the Codex Juris Gentium Diplomaticus, in which Leibniz claims that the two lowest degrees of right can be interpreted as ‘limited to the relations within mortal life’ since strict right ‘arises from the principle of preserving peace’ and equity can be justified as increasing one’s own happiness in this life (A 4.5.62f./L 423). He provides a similar justification in “On the Common Notion of Justice”: The most pressing reason not to harm others is ‘the fear that someone will [otherwise] do the same to us,’ and a good reason to help someone in need is the fear ‘that men will hate us if we refuse them aid which does not inconvenience us at all, and if we fail to arrest an evil which is going to overwhelm them’ (NCJ 165/Riley 54).⁵¹ Treating others well will ultimately increase one’s own happiness. Leibniz argues that parallel reasons apply to cases in which one can easily do positive good to others, that is, increase their happiness (NCJ 166f./Riley 55). These reasons ‘will even obligate [people] to exercise themselves from childhood to practice the rules of justice, in order to acquire the habit of doing so, for fear of betraying themselves too easily, and of thereby harming themselves along with others’ (NCJ 170/Riley 58).

Thus, even atheists have strong reasons to act in accordance with ius strictum, equity, and charity—at least in most situations—and to cultivate virtuous habits. For Leibniz, this means that atheists have a moral obligation to act that way. In very extreme cases, in which acting justly comes at a great personal cost, an atheist would probably not have sufficient reasons to be virtuous and will probably lack a sufficiently strong virtuous habit (see, e.g., NCJ 170/Riley 58; “Principles of Pufendorf” Dutens 4.3.277/Riley 67). This is presumably the reason why Leibniz sometimes says that piety perfects the other two degrees of right (Nova Methodus 2.75, A 6.1.344; NCJ 177/Riley 63); once we take God into consideration, we have stronger reasons to act in accordance with ius strictum, equity, and narrow charity. Nevertheless, the normative force of these principles is to a large extent independent of theological considerations; even atheists are obligated to obey the principles of equity and charity. This is important for determining the extent to which Leibniz’s argument against slavery depends on religious premises.

We can now investigate what implications the principles of equity, charity, and piety have for the institution of slavery. Leibniz argues that even if chattel slavery were not against nature, the rights of a slaveholder would be

limited by equity, which requires that a man take care of another man in the way that he would wish that others take care of him in a similar case, and by charity, which ordains that one work for the happiness of others. And these obligations are perfected by piety, that is, by what one owes to God. (NCJ 177/Riley 63)

In other words, the principle of equity requires slaveholders to treat their slaves in accordance with the Golden Rule. At the very least, this rules out excessive cruelty against slaves; but it arguably goes much further than that. The slaveholder must not do anything to her slaves that she would complain about if she were in their situation. This clearly requires much more than merely refraining from doing positive harm. In fact, as this passage states explicitly, charity requires that slaveholders aim for the happiness of slaves. If the slaveholder is a theist, they have particularly strong reasons to obey these principles and are additionally required to act piously. Yet, as seen earlier, even atheists are obligated, and have strong reasons, to act equitably and charitably.

Hence, on Leibniz’s theory of justice, an absolute right of slaveholders over slaves is morally impermissible because the three degrees of right restrict what a slaveholder is morally permitted to do. The rights of slaveholders are at best very limited: not only are they forbidden from causing slaves physical harm; they are not allowed to act in ways that will make the slaves unhappy and must always take the slaves’ happiness into consideration.⁵² In fact, whenever it is not too costly to do so, they must promote the slaves’ well-being. Hence, slaveholders cannot have absolute rights over slaves and must not treat slaves like property.

Even though Leibniz does not say so, the slaveholder’s duty to promote the slaves’ happiness might have much more radical implications than merely placing limits on how slaveholders can treat slaves. Arguably, Leibniz holds that self-determination, or freedom, increases happiness and may even be a necessary condition for happiness. The only type of subjugation that is compatible with true happiness, as seen in section 3, is the subjugation to a perfectly benevolent ruler like God. That kind of rule enables us to be happier and more virtuous than we would otherwise be, and we obey the will of such a ruler freely and without coercion. Since it is dubious that a slaveholder could rule her slaves in that way while maintaining their status as slaves, the slaveholder’s duty to promote the happiness of her slaves might require her to free them. Leibniz does not explicitly draw this consequence and it is unclear whether he endorses it, but it would make sense for him to draw it.

Toward the end of this paragraph, Leibniz brings up cannibalism as a reductio ad absurdum of the claim that one human being can have an absolute right over another. With this, he aims to address a potential opponent who denies that we are obligated to obey the principles of equity, charity, and piety, and who accepts only the ius strictum. If there were truly no limitations on the rights of slaveholders through moral principles like equity and charity, and if it amounted to full ownership, there would be nothing wrong with eating one’s prisoners or their children, just as one may eat one’s livestock. This, Leibniz implies, is clearly absurd, but ‘such are the consequences of the pretended absolute right of masters over slaves, or of fathers over children’ (NCJ 177/Riley 63).

5. Layer 3: Leibniz on Aristotelian Natural Slaves

The third and final layer of the argument occurs in a paragraph in which Leibniz advances the following argument: if, as already shown, chattel slavery is impermissible, it is a fortiori true that children are not the property of their parents. The paragraph is concerned mainly with the status of children, which makes sense given Leibniz’s aim of responding to the objection we identified in section 2. Nevertheless, this paragraph also contains some important claims about slavery. Interestingly, Leibniz takes a slightly different angle here: instead of asking whether the ownership of slaves and children is compatible with natural rights or with the principles of equity and piety, he asks whether it is overall most fitting. Thus, instead of focusing on the rights of individuals or the obligations toward individuals, he focuses on whether slavery ‘would suit the best government’ or might be ‘most conducive to order’ (Riley 63/NCJ 178).⁵³

To answer the question of what would be most conducive to order, Leibniz approvingly mentions Aristotle’s claim that the ownership of another person is fitting only in cases in which this other person is incapable of conducting herself well and therefore deserves to be someone else’s property. This is a reference to Aristotle’s theory that there are natural slaves, that is, human beings who lack reason and for whom it is therefore natural and expedient to be slaves (Politics I.4–5; I.13). Leibniz appears to agree that human beings who cannot conduct themselves well would be natural slaves. Yet, he ultimately concludes that children clearly do not fall into this category because they have the capacity to learn how to conduct themselves well. One should always assume that children, given the right sort of education, can become as virtuous and rational as their parents (NCJ 178/Riley 63).⁵⁴ Leibniz again refers to Aristotle here, namely to his distinction between paternal rule and despotic rule (regnum paternum and regnum herile). The goal of paternal rule is ‘to make subjects happy and virtuous,’ while it is the goal of despotic rule merely to preserve ‘a condition which makes [the subject] fit to work for their master’ (NCJ 178/Riley 63). Parents, unlike despots, have to try to make those under their rule happy and virtuous. This allows Leibniz to respond to the objection that motivated this entire discussion: it is not the case that if slaveholders have ownership rights over their slaves, then parents a fortiori have ownership rights over their children. Quite to the contrary: parental rule is different from despotic rule since they have completely different aims.

Thus far, it may sound as if Leibniz agrees with Aristotle that although children are not generally natural slaves, some human beings are. Yet, Leibniz adds one final and somewhat obscure sentence to this paragraph, which can be read as suggesting that there are no natural slaves among human beings at all. This sentence reads as follows: ‘But it seems that when one can make men happy and virtuous, one should never leave virtue out, although virtue has its degrees, and the same virtues are not necessary to [men in] all conditions, to make a man happy’ (NCJ 178/Riley 63). What does that mean? On one plausible interpretation, this suggests that masters must always promote the virtue and happiness of their subjects, at least to some degree. In other words, there is no example of a legitimate despotic rule as Aristotle describes it,⁵⁵ and thus perhaps there is no example of genuine natural slaves. Yet, this sentence is far from clear.

Luckily, there is another text in which Leibniz addresses the question of whether there are legitimate instances of despotic rule and of natural slaves. It is a German text titled “Divisio Societatum,” which the editors of the Akademie edition date to 1680. This text consists of Leibniz’s notes on the Latin book Elementa juris universi et in specie publici Justinianæi by Johann von Felden (1664).⁵⁶ Portions of Leibniz’s notes are summaries and paraphrases of the first few sections of von Felden’s book; other portions are Leibniz’s own comments and additions.⁵⁷ In the sections of the book that Leibniz focuses on, von Felden distinguishes in a very Aristotelian way between different types of natural human societies, most importantly the conjugal society between man and wife, the despotic society (Herilis societas) between master and slave, and the paternal society between parent and child (von Felden 1664: 2f.; Leibniz’s summary is in A 4.3.909). Leibniz spends the most time discussing the despotic society; what he says about it goes far beyond von Felden’s text.

According to von Felden, there are some human beings who completely lack the capacity to make intellectual judgments; they act, like animals, guided merely by their imagination and appetite (1664: 3). These human beings are natural slaves because it is proper for appetite to be guided by intellectual judgments; since these human beings lack intellectual judgment, it is proper that another person, who can judge intellectually, guides them. Indeed, von Felden goes so far as to say that the slave becomes as it were a second body of the master, and the master acts as the intellect of the slave (1664: 3). Thus, von Felden seems to view the master-slave relationship as an instance of the type of domination that we encountered in section 3, which holds between a rational soul and its body.

Leibniz summarizes von Felden’s points about natural slaves and then adds his own assessment. First, he says that this applies only to cases in which there is no hope that the slave might attain understanding; when there is such hope, the slaveholder is obligated to promote the slave’s freedom, insofar as it is needed for the slave’s happiness, through education (A 4.3.909/L 428). Next, Leibniz raises an even deeper worry about von Felden’s analysis:

I doubt whether an instance can be found of a servitude such that the servant⁵⁸ exists entirely for the sake of the master [as the body exists entirely for the sake of the soul], especially since souls are immortal and hence can sometime achieve understanding and the blessedness of a life based upon it. In my opinion, therefore, this society exists only between men and livestock [Vieh]. For even if a man were born entirely brutish [ganz thum] and incapable of any instruction, we should still not have the right to martyr, kill, or sell him to barbarians for our own good. (A 4.3.909f./L 428, translation altered)

Leibniz, in short, doubts whether there really are any human beings who are permanently incapable of attaining understanding. Combined with the previous point, this would mean that there cannot be slavery in von Felden’s sense; slaveholders always have the obligation to work toward the happiness and virtue of their subjects. In other words, human beings must not be treated like livestock. The reason Leibniz cites here is theological: humans have immortal souls, which are destined by God to become happy and virtuous. Even if they do not currently seem capable of understanding or judging rationally, they must possess that capacity and will actualize it eventually. As the last sentence indicates, Leibniz takes this to imply that we have at least certain negative obligations toward such human beings. We must not kill them, for instance, or place them into a situation in which their lives (and perhaps their salvation) are in danger. Moreover, as we saw earlier, he also holds that the slaveholder has the positive obligation to promote the slave’s happiness and freedom, if the slave is capable of attaining understanding—which we must assume.

Interestingly, Leibniz then goes on to consider whether things would be different if human souls were not immortal. His answer is revealing: he says that in that case, there might be entire peoples [Völcker] that are kept as unintelligent as livestock and that are therefore in servitude to other, more intelligent people.⁵⁹ Hence, when setting aside the theological doctrine of the immortality of souls—that is, in the context of discussing universal rules of justice which even atheists must accept—slavery can be natural, insofar as there are in fact human beings who are as unintelligent as animals (A 4.3.910/L 428f.). Leibniz concludes: ‘natural servitude applies to unintelligent men insofar as it is not restricted by the rules of piety [gottesfurcht]’ (A 4.3.910/L 429). In short, he is quite clear that what makes slavery impermissible are ultimately theological commitments. When setting aside such commitments, it is permissible to enslave individuals or even entire peoples who are unintelligent.

We should of course be somewhat cautious when using this much earlier text to help us interpret “On the Common Notion of Justice.” It is entirely possible that Leibniz changed his mind about various issues during that period. For instance, he may have changed his mind about whether piety, or the immortality of souls, is ultimately the only reason to reject the Aristotelian justification of slavery. As we saw earlier, Leibniz acknowledges some non-theological reasons to reject slavery in “On the Common Notion of Justice,” such as the natural right to freedom, the principles of equity and charity, and metaphysical doctrines about the relationship between the body and the soul.⁶⁰ While theological considerations play a major role even in “On the Common Notion of Justice”—as seen earlier, Leibniz relies on the claim that God is the rightful sovereign of human beings and that all humans are equal co-citizens in the City of God—Leibniz’s argument is quite strong even without those claims. Yet, even if Leibniz changed his mind in some respects, the content of “Diviso Societatum” makes it unlikely that when writing “On the Common Notion of Justice,” Leibniz fully embraces the Aristotelian doctrine that there are human beings who are natural slaves. Presumably, he still believes in 1703 that there are at least theological reasons to assume that all human beings are capable of rationality, virtue, and happiness.

Leibniz mentions further reasons for the assumption that all human beings are capable of rationality in another text, written right around the same time as “On the Common Notion of Justice”: the New Essays. Leibniz there uses a definition of ‘human’ in which a possession of the faculty of reason is a necessary condition (e.g., NE 313). This means that a natural slave—that is, someone completely incapable of intellectual judgment—would by definition not count as human, even if they looked like a human being. A definition like this does not get us very far, of course; it might even be used to justify the enslavement of those who are incapable of intellectual judgment. Yet, Leibniz then makes an interesting epistemological point about situations in which someone has a human form but does not exhibit any signs of rationality: since we cannot be sure that they genuinely lack the capacity for rationality, we should always presume that they have it, even if it is not currently manifested and even if they are at an age at which humans typically manifest the faculty of reason (NE 395). In other words, he advocates erring on the side of caution, since ‘if this human-shaped animal is not a man, no great harm will come from caring for it while we are uncertain about its fate’ (NE 395). Conversely, Leibniz seems to imply, treating someone like a brute animal even though they are in fact human would be immensely harmful and hence immoral. This means that even if there are natural slaves, we can never know this for certain and should never treat anybody as such. As a result, Leibniz appears committed to the view that the Aristotelian doctrine of natural slaves cannot be used to justify enslaving anyone, no matter what intellectual abilities they manifest.

6. Conclusion and Evaluation of Leibniz’s Argument

As we have seen, Leibniz’s argument against chattel slavery has three layers. The first is that slavery is opposed by three natural rights: the inalienable right of rational souls to be free, the right of God to govern his creatures, and the right of rational souls to be citizens in God’s moral kingdom. Moreover, a person’s body can only be owned by the person’s own soul, and souls in turn cannot be acquired. The second layer is that the principles of equity, charity, and piety oppose slavery, or at least severely limit the slaveholder’s rights over her slaves. The third and final layer is that slavery can only be justified if the slave is permanently incapable of conducting herself well. However, it is doubtful that any actual human beings satisfy that description, for Leibniz.

Leibniz’s argument against slavery is not as clearly laid out as one might have hoped. This is due in part to the context of Leibniz’s discussion: his main goal in this text is not to examine the moral status of slavery, but rather to respond to an objection concerning the rights of parents over their children. Slavery is only of secondary importance in this text. Leibniz does make important claims about the moral status of slavery along the way that have the potential to be quite powerful, but he does not spell out their implications in much detail and makes some points only indirectly. It takes quite a bit of interpretive work to reveal the force of Leibniz’s argument.

Nevertheless, “On the Common Notion of Justice” is without question very important for Leibniz scholarship: it is an excellent source of information on Leibniz’s moral philosophy and his views on the moral status of slavery. Moreover, if my interpretation is correct, this text puts forward several strong reasons to view chattel slavery, and arguably even other forms of slavery, as morally impermissible. Given how widely accepted slavery still was in Leibniz’s philosophical and social circles, this is noteworthy. The strongest of these reasons is, as seen, that rational beings have a special moral status that confers on them a number of rights, such as the right to freedom and membership in the City of God. Conversely, there are strict moral obligations governing the treatment of rational beings: not only is it impermissible to interfere with their freedom, but it is obligatory to promote their virtue and happiness insofar as one can. To put this slightly differently, it is not permissible to use rational beings for one’s own ends without any concern for their good. As I hope to have shown, these claims about moral rights and obligations are premises in a powerful argument against the moral permissibility of slavery.⁶¹

Abbreviations

If an English edition is explicitly cited (or associated with an abbreviation), translations are taken from that English edition unless otherwise specified. In all other cases, translations are mine. Quotations include all italics from the original text, unless otherwise specified.

Competing Interests

The author has no competing interests to declare.