A Thomistic Reflection on John Locke’s Theory of Natural Law
Peter P. Cvek, Associate Professor, Saint Peter’s University, Jersey City, New Jersey USA
pcvek@saintpeters.edu
ABSTRACT
This paper is a reflection on Locke’s theory of natural law from the perspective of contemporary
interpretations of Thomas Aquinas’ understanding of the relationship between natural law and natural
inclinations. At the heart of the natural law tradition is the belief that moral principles or basic human
goods are rooted in nature, or more precisely, that these principles can be discovered by rational reflection
on those natural inclinations which constitute the core of human nature. Every natural law theory assumes
that there is a necessary connection between Nature and Reason. Traditional theories will include God,
while modern versions may adopt a more secular approach. My concern is with Locke’s theory of the
natural law, more specifically, on how Locke understands the crucial relationship between natural law and
natural inclinations. Moreover, I intend to examine Locke’s comments on natural law from a Thomistic
perspective. First, I review three contemporary interpretations of the relationship between natural law and
natural inclinations based on Aquinas’ Treatise on Law. Second, I re-examine Locke’s theory of natural
law in light of the aforementioned interpretations of Aquinas. While Aquinas forges a synthesis between
nature, reason, and God, I will show that in his struggle to produce a “modern” version of natural law
Locke was compelled to adopt a theologically-based conception of natural law, which subordinates both
nature and reason to God and revelation.
A Thomistic Reflection on John Locke’s Theory of Natural Law
At the heart of the natural law tradition in ethics is the belief that moral principles, or basic
human goods, are rooted in nature, or from an epistemic point of view, that these principles can
be discovered by rational reflection on natural inclinations, those inclinations which constitute
the core of human nature. Every natural law theory therefore assumes that there is a significant
connection between Nature and Reason. Traditional theories will also take for granted that there
is some relationship between natural law and God.
My primary concern in this paper is with John Locke’s theory of the natural law, more
specifically, on how Locke understands the relationship between natural law and natural
inclinations. This will, of course, also shed light on the role played by Reason and God. But
while it is common to interpret Locke’s thought from within the intellectual context of other 17th
century natural law theorists, in this paper, I intend to re-examine Locke’s often fragmented
comments on natural law from the perspective of contemporary readings of Thomas Aquinas’s
discourse on law.
In the first part of the paper, I will briefly review three recent interpretations of the
relationship between natural law and natural inclinations in Aquinas. I will also identify how
each interpretation construes the relationship between the Nature, Reason, and God. In the
second part, I will re-examine Locke’s theory of natural law in light of the aforementioned
interpretations of Aquinas. While the Thomistic theory of natural law represents a synthesis of
these three elements into a comprehensive unity, I will argue that in his struggle to produce a
“modern” theory of natural law Locke, perhaps ironically, was compelled to construct a
theologically-based conception of natural law, which subordinates both Nature and Reason to
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God and revelation. Nevertheless, despite their differences, the natural law theories of Aquinas
and Locke can be seen to be part of a broader Christian natural law tradition which views
morality as a function of the relationship between Nature, Reason, and God.
Thomas Aquinas on Natural Law and Natural Inclinations
One of the hallmarks of the natural law tradition is the attempt to ground morality in
human nature, or more precisely, natural law ethics is based on the belief that there is a necessary
connection between natural inclinations and the principles of natural law, a connection mediated
by human reason. This thesis is succinctly captured by Thomas Aquinas in the Question 94, art.
2 S.T. I-II. After distinguishing between theoretical and practical reasoning, Aquinas states that
the first principle of practical reasoning is founded on the nature of the good, since the good is
that which all things desire. Therefore, the first principle of natural law is that good is to be done
and pursued, and evil is to be avoided. All the other principles (or precepts) of the natural law
are based on this formal principle, for all those things which reason naturally apprehends as good
belong to the natural law. More importantly, this apprehension of the good, and therefore the
order of the natural law principles, is said to be based on the order of natural inclinations, for “all
those things to which man has a natural inclination are naturally apprehended by reason as being
good, and consequently as objects of pursuit, and their contraries as evil, and objects of
avoidance” (I-II. 94, 2). By reflecting on the order of natural inclinations, Aquinas arrives at four
fundamental principles (or basic human goods) of the natural law, which can be listed as the
preservation of life, the care and education of children, knowledge, and the good of an ordered
social life. Each is these goods is said to be connected to, or inferred from, a specific natural
inclination.
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The problems with the most obvious reading of this derivation, however, are well known.
Aquinas seems to suggest that the mere rational apprehension of an ‘inclination to x’ reveals to
reason that ‘x is good’. But why should that be the case? Does this apply to every inclination,
including those that appear to be aimed at evil, or only to those inclinations apprehended as
promoting some good? And if the latter, then of what use are the inclinations, assuming that the
inclinations themselves need to be evaluated by reason? In any case, the precise nature of the
relationship between natural inclinations and natural law is not explained here and has generated
considerable controversy among contemporary proponents of a Thomistic theory of natural law
(see Brock, 2005).
Fortunately, the natural law tradition itself provides a useful typography for
understanding this problem. Philosophical reflection on natural law has ultimately been a
function of attempting to understand the relationship between three fundamental realities:
Nature, Reason, and God. While all three elements play a role in any natural law theory,
significant differences in emphasis will distinguish one interpretation from another. As luck
would have it, we have contemporary examples of each approach.
Lisska: Nature
The interpretation grounded primarily in Nature maintains the very tight connection
between natural inclinations and natural law principles which is seemingly implied by Aquinas
in Question 94. Anthony Lisska’s Aquinas’s Theory of Natural Law: An Analytic Reconstruction
(1996) is one of the best contemporary defenses of what might be called the “traditional”
reading. This Aristotelian inspired approach grounds reason’s apprehension of natural law in a
teleological understanding of human nature and inclinations (Lisska, 99). On this reading, the
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ends that identify the human telos are objectively good, not only because they are the objects of
inclinations, but because their achievement contributes to the optimal functioning (or perfection)
of any human being. Hence, the goods associated with the natural law are essential to the full
actualization of human potential and the achievement of the highest good, described variously as
well-being, human flourishing, perfection, or happiness.
Finnis: Reason
In contrast, other commentators have argued that our knowledge of the natural law lies
not in reflection on natural inclinations, but in reason’s own understanding of the good. What
has become known as the ‘new natural law theory’ has been developed most fully by John Finnis
in his Natural Law & Natural Rights (1980) and more recently in Aquinas (1998). Finnis insists
that the principles of natural law are grasped by reason as self-evident (per se nota) and
indemonstrable (Finnis, 1980, 33-34). On this reading, practical reason’s apprehension of the
good is logically prior to any theoretical understanding of human nature. For Finnis, the rational
apprehension that the object of this inclination is a good does not consist in simply observing that
such an inclination happens to exist. Rather, the role of reason is (to paraphrase Brock, 59) to
“make the inclination itself intelligible” by rationally demonstrating that the object of this
inclination is something valuable for its own sake. And, while it may seem strange to have a
theory of natural law sans nature, Finnis concedes that the experience of inclinations and their
objects might provide the raw material for reflection, but it is the task of reason alone to
apprehend what is good. He also assumes that these goods will as a matter of fact be the objects
of natural inclinations, although this appears to be more of a happy coincidence, or an
arrangement made possible by God, as Finnis (1998) later speculates.
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Porter: God
Much of the contemporary debate over natural inclinations and natural law has taken
place in the space between these two approaches, with different commentators finding different
levels of interplay between nature and reason. Most of these accounts have continued the modern
emphasis on constructing a philosophical conception of natural law which is independent of any
theological commitments. It is worth noting that metaphysically speaking, both Lisska and Finnis
agree that for Aquinas natural law is grounded in the eternal law in the mind of God, but they
also insist that appeals to God or divine revelation play no epistemic role in explaining our
apprehension of natural law. In this sense, both interpretations are naturalistic in their approach
to knowing the natural law. One exception is found in Jean Porter’s Natural Law and Divine
Law: Reclaiming the Tradition for Christian Ethics (1999) and Nature as Reason: A Thomistic
Theory of Natural Law (2005). Porter defends what she calls a “theological” conception of
natural law. According to Porter, the scholastic understanding of natural law was primarily based
on the study of various texts, with Scripture being authoritative (Porter 2005, 8). Within this
project, Nature was fundamentally understood through the theological categories of creation and
providence (Porter 2005, 65). So that even when human beings were grasped by reason in terms
of the inclinations inherent in their nature, they remained “creatures” in and through which God
manifested his creative power and wisdom (Porter 2005, 69).
In terms of the relationship between natural law and natural inclinations, Porter partially
agrees with the traditional interpretation: the scholastics did ground their accounts of natural law
in “a robust concept of nature,” but they also took for granted that “nature underdetermines
morality” (Porter 2005, 126). Hence, it was necessary to view human inclinations within a
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broader theological context. It was this Scriptural-based understanding of human nature that
provided “the necessary framework within which reason move(d) from reflection on human
inclinations to a developed account of natural law morality” (Porter 2005, 80). While Aquinas
assumed that reason and revelation will ultimately agree in revealing the natural law, it is
important to note that Porter’s thesis is stronger than that. According to Porter, Scripture has a
necessary epistemic role to play in facilitating our human knowledge of natural law that is not
found in the aforementioned naturalistic interpretations. On her reading, our apprehension of
natural law is not based on unaided reason alone (Finnis), nor on unaided reason reflecting on
natural inclinations (Lisska), but reason informed by revelation. In this sense, Porter’s
conception of natural law is decidedly theistic in character, as opposed to the naturalistic
interpretations advanced by Finnis and Lisska.
Each of these three interpretations provides a distinct way of understanding the
relationship between natural law and natural inclinations, as well as the interplay between
Nature, Reason, and God. While each of these approaches is reflected in Locke’s natural law
project, I will show that Porter’s theological and Scriptural account of natural law has much to
recommend it, not perhaps as the definitive reading of Thomas Aquinas, but as an illuminating
way of understanding the development of John Locke’s conception of natural law.
John Locke on Natural Law and Natural Inclinations
At first blush, Locke’s conception of natural law seems far removed from that advanced
by Thomas Aquinas. In the great debate between the intellectualist and voluntarist theories of
law, Aquinas is the quintessential intellectualist, while Locke is clearly committed to some form
of voluntarism. Upon closer examination, Locke most likely adopted a variation of the so-called
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“middle course” between intellectualism and voluntarism associated with Francisco Suarez and
Nathaniel Culverwell (Tully 1980, 41). This conception of law is best illustrated in his early
Essays on the Law of Nature. In the Sixth Essay, Locke states that the formal cause of a law is
the will of a superior (185). God, or more precisely, God’s will, is the cause of the morally
obligatory character of the natural law. But, if God’s will is the formal cause, then human nature
is the material cause, for it is only by rational reflection on human nature that one is able to know
what God has commanded. For this reason, in the Seventh Essay, Locke describes the natural law
as “a fixed and permanent rule of morals, which reason itself pronounces, and which persists,
being a fact so firmly rooted in the soil of human nature” (199). Based on this distinction, Locke
describes the natural law as “the decree of the divine will discernible by the light of nature and
indicating what is and what is not in conformity with rational nature, and for this very reason
commanding or prohibiting” (111). In so far the content of the natural law is “discernible by the
light of nature,” i.e., reason, which is able to indicate “what is and what is not in conformity with
rational nature,” it would appear that, as with Aquinas, the key to understanding Locke’s theory
of natural law depends on determining how Locke construed the crucial relationship between
natural inclinations and natural law. This would also seem to commit Locke to a naturalistic
approach to natural law, despite his theistic and voluntarist account of moral obligation.
Samuel Zinaich has recently argued that, in his early Essays, Locke assumed the validity
of the Aristotelian world-view. If true, this would seemingly commit Locke to the traditional
understanding of the relationship between natural inclinations and natural law. On the one hand,
Zinaich’s defense of an Aristotelian reading of the Essays is certainly plausible given Locke’s
claim to ground natural law in human nature. While demonstrating the existence of the natural
law, Locke cites both Aristotle and Aquinas (Essays, 113, 117), and while defending the good of
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social life, Locke claims that all human beings are “urged to enter into society by a certain propensity
of nature” (Essays, 157). Zinaich compares this favorably to Aquinas’ derivation of natural law
principles from natural inclinations (39). On the other hand, there is considerable evidence that
the aforementioned reference to a natural propensity is the exception to the rule. Locke
continuously warns that natural appetites, desires, or inclinations are more likely to run contrary
to the moral law than provide guides to right conduct, and that it would be a mistake to hold that
the “principles of moral action and a rule to live by” can be found “in men’s appetites and natural
instincts …just as if that was morally best which most people desired” (Essays, 213). So, while
Locke does believe that there are principles of action inherent in human nature, he seems to
believe that they cannot be the basis for our understanding of morality.
Moreover, when he has the opportunity to affirm that the natural law can be known by
rational reflection on natural inclinations, Locke explicitly denies it. This denial is all the more
intriguing in that it occurs only in the title of an unwritten essay, “Can the Law of Nature be
Known from Man’s Natural Inclinations? No. According to Zuckert, this title of an unwritten
essay is Locke’s definitive rejection of any relevant connection between natural inclinations and
natural law (201). There is no way of knowing why Locke never completed this essay. Zinaich’s
suggestion that the essay would be redundant, merely repeating the denial that the natural law is
inscribed (innate) in the minds of men contained in the Third Essay, is unconvincing, since a
natural inclination is not equivalent to an innate idea, and Locke certainly affirms the former,
while denying the latter (73). However, his explanatory note, to the effect that there is no way to
discriminate between inclinations which dispose us to do our duty from those that lead us astray
without “some sense of who God is,” is more telling (Zinaich 73, note 37).
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It would seem that for Locke without some knowledge of God, rational reflection on
human nature alone is insufficient to reveal the principles of natural law. The exact nature of the
role played by God, however, is not fully explained here. Nevertheless, this appeal to God might
provide a response to the most decisive objection to Locke’s defense of natural law: his rejection
of natural teleology. There is no doubt that Locke eventually rejects the Aristotelian worldview,
in favor of the corpuscular theory of nature, according to which the real essences of things are
configurations of matter in motion, explainable in terms of efficient causes. It is likely that Locke
had his suspicions even while writing the Essays. On the traditional reading, Locke’s rejection of
Aristotelianism, especially the idea of final causality, is tantamount to a rejection of the natural
law doctrine itself (Feser 110).
Although Robert Boyle does provide a space for teleological explanations in an otherwise
mechanical understanding of nature, Locke appears reluctant to follow Boyle on this matter.
While Locke agrees that Nature exhibits purposive behaviors, and that these purposive behaviors
are the result of God’s intelligent design, his skepticism dissuades him from seeking teleological
explanations in nature alone. Locke doubts our ability to grasp the real essences of things. Some
understanding of the nominal essence might be provide a starting point for reflection on
purposes, but the goods of the natural law cannot be inferred from a simple examination of
natural inclinations alone. Locke cautions against placing too much trust in these pre-rational
inclinations and seems to agree with Finnis’ contention that reason would have to determine
which of our many inclinations and desires are worthy of pursuit. However, in contrast with
Finnis, and despite his own promise of a rational demonstration of morality, Locke is more
inclined to appeal to God as the ultimate source of this knowledge. So, if Locke’s theory of
natural law is to succeed, then it is not natural inclinations as such, but God’s will for his creation
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that is expressed in and through these inclinations that is the ground of our knowledge of natural
law, for it is only by viewing these inclinations in the context of God’s reason and creative will
that they become fully intelligible.
The addition of this theological prerequisite to our understanding of natural law is
indicative of Porter’s interpretation of Aquinas. But it also raises the issue of the relationship
between Reason and Revelation. It is therefore not surprising that while this connection between
God and morality has been acknowledged by recent commentators, for example, Richard
Ashcraft (1987, 38) and Ian Harris (1994, 98), there is considerable disagreement over just how
much Locke is able to accomplish within this theological framework.
The problem is determining how one comes to know these divine purposes and how this
insight into the divine mind relates to our experience of natural inclinations. Locke rejects innate
knowledge, and he does not believe that we have any special access to the mind of God, what
Locke calls “enthusiasm” (Essay 4.19.6-13). He insists that the natural law can be known by a
combination of reason and experience. He also takes for granted that reason and revelation will
agree, but typically insists that reason is our best guide to understanding what has been revealed
in Scripture (4.18.8). Locke’s strategy appears assume that even if the real essence of human
nature is unknown to us, our knowledge of the nominal essence, in this case, our experience of
specific natural inclinations, in conjunction with our recognition of God’s intention in creating us
with these inclinations, is sufficient for our knowledge of the natural law. Given this approach,
however, it is not surprising that appeals to Scripture will increasingly play a role in Locke’s
understanding of natural law. What I hope to make clear is that Locke’s solution is eclectic at
best. God’s will may be revealed in multiple ways, including data provided by rational reflection
on natural inclinations, analysis of the idea of God as creator of the natural order, what reason
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and experience tells us about the human condition, as well as significant clues provided by
Scripture.
Locke’s Derivation of the Principles of Natural Law
Throughout the Two Treatises, Locke repeatedly declares that the fundamental law of
nature commands the preservation of mankind, both individually and collectively. In a revealing
passage, Locke states that this law of preservation is based on the inclination for selfpreservation placed in human nature by God, such that “Reason, which was the voice of God in
him, could not but teach him and assure him, that pursuing that natural Inclination he had to
preserve his Being, he followed the will of his Maker. (1.86)
But, how does reason ascertain the connection between God’s will and the desire for selfpreservation? Locke’s strategy is two-fold. What might be called the a priori argument
expounds the logic of God’s creative act. That the inclination toward self-preservation is in
accordance with God’s will is evident from the idea of creation itself. Since we are all created
and preserved by God, it is obvious that God intends that we live. In other words, it would be
inconsistent for God to bring human beings into existence with a strong desire for selfpreservation and at the same time not wish that they continue to existence (see also Forde, 2001).
This a priori argument is combined with an a posteriori argument which reflects on the human
condition and the circumstances surrounding creation itself. For example, the divine intention
that humanity be preserved is further indicated by the fact that God “furnished the world with
things Fit for Food and Raiment and other Necessaries of Life, Subservient to His design, that
man should live and abide for some time upon the face of the Earth” (1.86). Both arguments
make the natural inclination for self-preservation intelligible as an indication of God’s rational
will.
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But, If everyone has a duty to seek the preservation of their own life, what about the duty
to preserve mankind in general? Locke declares that “Everyone as he is bound to preserve
himself…so by the like reason when his Preservation comes not in competition, ought he, as
much as he can, to preserve the rest of mankind” (2.6). Locke says “so by the like reason,” the
duty to preserve mankind follows from the duty to preserve oneself, but never elaborates on this
comparison. If the like reason is the existence of a natural inclination or desire, the problem is
that of ascertaining whether human beings are so inclined. If anything, Locke’s descriptions of
human nature emphasize our tendencies to pursue our own self-interest. If the like reason is that
it is willed by God then the duty is not so much grounded in human nature, but in divine fiat; in
which case Locke’s natural law collapses into divine law, knowable, if at all, by revelation.
One promising solution to this dilemma is suggested by Locke himself. After discussing
the freedom and equality of all men in the state of nature, Locke quotes a passage from Hooker,
who suggests that all of our moral duties can be derived from one’s duty to love one’s neighbor
as one’s self (Harris 219). This might be called “the golden rule” solution. According to Ian
Harris, Locke combined the desire for self-preservation with the golden rule, or its Biblical
equivalent, the commandment to love thy neighbor as thyself, in order to derive the duty to
preserve mankind in general. (Harris 219-222). It is worth noting that this strategy resembles
Aquinas’ own use of the golden rule (ST I-II, 100, 3). As Rhonheimer explains, the golden rule
transforms “what is ‘good for me’ into something that could be a good owed to others as well,
changing it into the fundamental structure of human fellowship” (2011, 285). For both Locke and
Aquinas, then it would seem that the golden rule is the foundation for the duties we owe to one
another.
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But as one might expect, Locke’s endorsement of the golden rule is problematic. Arguing
against innate moral knowledge in the Essay, Locke insists that “there cannot any one moral rule
be proposed whereof a man may not justly demand a reason,” and cites the golden rule as a
prime example of a rule that requires justification. At the same time, he refers to it as “that most
unshaken rule of morality and foundation of all social virtue” (1.2.4). Colman concludes that
Locke never provided “a satisfactory reason” for accepting the golden rule and that this explains
his failure to produce a demonstrative system of morality (C 204). Harris suggests that Locke
simply took for granted the importance of this and other moral rules found in Scripture (H 219).
Both may be correct. Ideally, Locke sought to place morality on a rational foundation, but as he
concedes in the Reasonableness of Christianity, reason has not proven up to the task. The
acceptance of the golden rule therefore is a case of reason informed by revelation.
This convergence of reason and revelation is most evident in Locke’s discussion of the
natural law precept that commands the propagation of the species. Again Locke cites a natural
inclination as an indication of God’s will. So while “the first and strongest desire God Planted in
Men, and wrought into the very Principles of their Nature (is) that of Self-preservation,” next to
this “God planted in Men a strong desire also of propagating their kind, and continuing
themselves in their Posterity” (1.88). We are, of course, faced with the same difficulty, how do
we determine that this particular inclination is an indication of God’s will, especially when, as
Locke points out (1.54), this desire is so often pursued only for the sake of the immediate
pleasure associated with it.
It is, perhaps, here more than anywhere else, that revelation comes to the aid of reason.
While arguing against Filmer’s claim that the earth was given by God only to Adam, Locke
maintains that this alleged donation would be inconsistent with what he calls “the great Design
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of God” (1.41). This great design is revealed in God’s command stated in Genesis: “And God
said unto them, be Fruitful and Multiply” (I Gen. 28). What God intends is obvious: “to promote
the great Design of God, Increase and Multiply” (1.41) [See also Harris, 215]. This divine
command makes intelligible not only the desire to procreate, but also the desire for selfpreservation and the duty to preserve mankind as whole, consistent with the logic of creation
itself. It also explains why the earth was given to humanity in common and not to a single
individual, since “it is reasonable to think, that God who bid Mankind increase and multiply,
should rather himself give them all a Right, to make use of the Food…and other Conveniences of
Life, the Materials whereof he had so plentifully provided for them” (2.6).
In order to further God’s great design, Locke also concludes that the preservation of
humanity requires the preservation of society (2.134). As Locke states in the Essay: “God having
designed Man for a sociable Creature, made him not only with an inclination, and under a
necessity to have fellowship with those of his own kind; but furnished him also with Language,
which was to be the great Instrument, and common Tye of Society” (3.1.1). And again, Locke
views this inclination, along with empirical evidence of necessity and natural sociability, as well
as the law of preservation, to be an indication of God’s rational will.
Conclusion
In seeking to develop a modern conception of natural law, Locke sought to defend a theory of
natural law that is compatible with the new science. By rejecting the Aristotelian worldview, the
traditional approach to grounding morality in Nature alone is unavailable to him. We have no
access to the real essence of humanity and even if we did, an understanding of the mechanics of
human nature would appear to be of little help in apprehending the purpose of human existence.
Of course, we could construct a conception of human nature based on our experience of
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ourselves. This nominal essence would include a variety of inclinations and desires, along with
reason, which allows us to reflect on these non-rational aspects of our being. These inclinations
are experienced as end-directed, regardless of our inability to grasp our real essence. But, in so
far as “nature underdetermines morality,” Locke’s natural law theory depends upon our ability to
recognize that some of these inclinations are directed toward ends, or natural goods, that are not
only desirable, but rationally willed by God and as such morally obligatory. In keeping with the
modern call for a science of morality, Locke frequently appeals to Reason as that which reveals
the natural law and declares that morality should be capable of demonstration. Unfortunately,
this demonstration is never forth coming. Since he is confident that God would not create human
beings who are capable of self-understanding and at the same time frustrate that goal, he insists
that reason and experience are capable of apprehending God’s intentions for humanity. It is these
intentions that ultimately make natural inclinations morally intelligible, not the reverse. Of
course, this ‘theistic project’ would require a fully developed natural theology, which Locke
considers but never completes.
Although Locke sought a natural law based only on reason and experience, throughout
his discussions of natural law he increasingly appeals to revelation to fill in the gaps. This is
quite evident, as we have seen, in the Two Treatises. In The Reasonableness of Christianity,
Locke finally concedes that reason has failed in this task, only to be set on the right path by
Scripture (137). Only the introduction of revelation has allowed Christian philosophers to
advance beyond their classical compatriots. This admission returns Locke to the Christian natural
law tradition envisioned by Porter, within which appeals to Scripture fill in what reason is unable
to attain on its own.
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