The main lines of Locke’s natural law theory are as follows: there is a moral law that is ( discoverable by the combined work of reason and sense experience, and binding on human beings in virtue of being decreed by God. Locke thinks that all human beings are naturally oriented to the pursuit of happiness. This is because we are motivated to pursue things if they promise pleasure and to avoid things if they promise pain. It has seemed to many commentators that these two discussions of moral principles are in tension with each other. On the view described in Law, Locke straightforwardly appeals to reason and our ability to understand the nature of God’s attributes to ground our obligation to follow the law of nature. In other words, what is lawful ought to be followed because God wills it and what is unlawful ought to be rejected because it is not willed by God. Because we can straightforwardly see that God is the law-giver and that we are by nature subordinate to Him, we ought to follow the law. By contrast, in the discussion of happiness and pleasure in the Essay, Locke explains that good and evil reduce to what is pleasurable and what is painful. While he does also indicate that the special categories of good and evil—moral good and moral evil—are no more than the conformity or disagreement between our actions and a law, he immediately adds that such conformity or disagreement is followed by rewards or punishments that flow from the lawmaker’s will. From this discussion, then, it is difficult to see whether Locke holds that it is the reward and punishment that binds human beings to act in accordance with the law, or if it is the fact that the law is willed by God.
One way to approach this problem is to suggest that Locke changed his mind. Because of the thirty-year gap between Law and the Essay, we might be tempted to think that the more rationalist picture, where the law and its authority are based on reason, was the young Locke’s view when he wrote Law. This view, the story would go, was replaced by Locke’s more considered and mature view, hedonism. But this approach must be resisted because both theories are present in early and late works. The role of pleasure and pain with respect to morality is present not only in the Essay, but is invoked in Law (passage quoted at the end of §2c), and many other various minor essays written in the years between Law and Essay (for example, ‘Morality’ (c.1677–78) in Political Essays, 267–69). Likewise, the role of the authority of God's will is retained after Law, again evident in various minor essays (for example, ‘Virtue B’ (1681) in Political Essays, 287-88), Government II.6), Locke’s correspondence (for example, to James Tyrrell, 4 August 1690, Correspondence, Vol.4, letter n.1309) and even in the Essay itself (II.xxviii.8). An answer to how we might reconcile these two positions is suggested when we consider the texts where appeals to both theories are found side-by-side in certain passages.
In his essay Of Ethick in General (c. 1686–88) Locke affirms the hedonist view that happiness and misery consist only in pleasure and pain, and that we all naturally seek happiness. But in the very next paragraph, he states that there is an important difference between moral and natural good and evil—the pleasure and pain that are consequences of virtuous and vicious behavior are grounded in the divine will. Locke notes that drinking to excess leads to pain in the form of headache or nausea. This is an example of a natural evil. By contrast, transgressing a law would not have any painful consequences if the law were not decreed by a superior lawmaker. He adds that it is impossible to motivate the actions of rational agents without the promise of pain or pleasure (Of Ethick in General, §8). From these considerations, Locke suggests that the proper foundation of morality, a foundation that will entail an obligation to moral principles, needs two things. First, we need the proof of a law, which presupposes the existence of a lawmaker who is superior to those to whom the law is decreed. The lawmaker has the right to ordain the law and the power to reward and punish. Second, it must be shown that the content of the law is discoverable to humankind (Of Ethick in General, §12). In this text it seems that Locke suggests that both the force and authority of the divine decree and the promise of reward and punishment are necessary for the proper foundation of an obligating moral law.
A similar line of argument is found in the Essay. There, Locke asserts that in order to judge moral success or failure, we need a rule by which to measure and judge action. Further, each rule of this sort has an “enforcement of Good and Evil.” This is because, according to Locke, “where-ever we suppose a Law, suppose also some Reward or Punishment annexed to that Law” (Essay, II.xxviii.6). Locke states that some promise of pleasure or pain is necessary in order to determine the will to pursue or avoid certain actions. Indeed, he puts the point even more strongly, saying that it would be in vain for the intelligent being who decrees the rule of law to so decree without entailing reward or punishment for the obedient or the unfaithful (see also Government, II.7). It seems, then, that reason discovers the fact that a divine law exists and that it derives from the divine will and, as such, is binding. We might think, as Stephen Darwall suggests in The British Moralists and the Internal Ought, that if reason is that which discovers our obligation to the law, the role for reward and punishment is to motivate our obedience to the law. While this succeeds in making room for both the rationalist and hedonist strains in Locke’s view, some other texts seem to indicate that by reason alone we ought to be motivated to follow moral laws.
One striking instance of this kind of suggestion is found in the third book of the Essay where Locke boldly states that “Morality is capable of Demonstration” in the same way as mathematics (Essay, III.xi.16). He explains that once we understand the existence and nature of God as a supreme being who is infinite in power, goodness, and wisdom and on whom we depend, and our own nature “as understanding, rational Beings,” we should be able to see that these two things together provide the foundation of both our duty and the appropriate rules of action. On Locke’s view, with focused attention the measures of right and wrong will become as clear to us as the propositions of mathematics (Essay, IV.iii.18). He gives two examples of such certain moral principles to make the point: (1) “Where there is no Property, there is no Injustice” and (2) “No Government allows absolute Liberty.” He explains that property implies a right to something and injustice is the violation of a right to something. So, if we clearly see the intensional definition of each term, we see that is necessarily true. Similarly, government indicates the establishment of a society based on certain rules, and absolute liberty is the freedom from any and all rules. Again, if we understand the definitions of the two terms in the proposition, it becomes obvious that is necessarily true. And, Locke states, following this logic, 1 and 2 are as certain as the proposition that “a Triangle has three Angles equal to two right ones” (Essay, IV.iii.18). If moral principles have the same status as mathematical principles, it is difficult to see why we would need further inducement to use these principles to guide our behavior. While there is no clear answer to this question, Locke does provide a way to understand the role of reward and punishment in our obligation to moral principles despite the fact that it seems that they ought to obligate by reason alone.
Early in the Essay, over the course of giving arguments against the existence of innate ideas, Locke addresses the possibility of innate moral principles. He begins by saying that for any proposed moral rule human beings can, with good reason, demand justification. This precludes the possibility of innate moral principles because, if they were innate, they would be self-evident and thus would not be candidates for justification. Next, Locke notes that despite the fact that there are no innate moral principles, there are certain principles that are undeniable, for example, that “men should keep their Compacts.” However, when asked why people follow this rule, different answers are given. A “Hobbist” will say that it is because the public requires it, and the “Leviathan” will punish those who disobey the law. A “Heathen” philosopher will say that it is because following such a law is a virtue, which is the highest perfection for human beings. But a Christian philosopher, the category to which Locke belongs, will say that it is because “God, who has the Power of eternal Life and Death, requires it of us” (Essay, I.iii.5). Locke builds on this statement in the following section when he notes that while the existence of God and the truth of our obedience to Him is made manifest by the light of reason, it is possible that there are people who accept the truth of moral principles, and follow them, without knowing or accepting the “true ground of Morality; which can only be the Will and Law of God” (Essay, I.iii.6). Here Locke is suggesting that we can accept a true moral law as binding and follow it as such, but for the wrong reasons. This means that while the Hobbist, the Heathen, and the Christian might all take the same law of keeping one’s compacts to be obligating, only the Christian does it for the right reason—that God’s will requires our obedience to that law. Indeed, Locke states that if we receive truths by revelation they too must be subject to reason, for to follow truths based on revelation alone is insufficient (see Essay, IV.xviii).
Now, to determine the role of pain and pleasure in this story, we turn to Locke’s discussion of the role of pain and pleasure in general. He says that God has joined pains and pleasures to our interaction with many things in our environment in order to alert us to things that are harmful or helpful to the preservation of our bodies (Essay, II.vii.4). But, beyond this, Locke notes that there is another reason that God has joined pleasure and pain to almost all our thoughts and sensations: so that we experience imperfections and dissatisfactions. He states that the kinds of pleasures that we experience in connection to finite things are ephemeral and not representative of complete happiness. This dissatisfaction coupled with the natural drive to obtain happiness opens the possibility of our being led to seek our pleasure in God, where we anticipate a more stable and, perhaps, permanent happiness. Appreciating this reason why pleasure and pain are annexed to most of our ideas will, according to Locke, lead the way to the ultimate aim of the enquiry in human understanding—the knowledge and veneration of God (Essay, II.vii.5–6). So, Locke seems to be suggesting here that pain and pleasure prompt us to find out about God, in whom complete and eternal happiness is possible. This search, in turn, leads us to knowledge of God, which will include the knowledge that He ought to be obeyed in virtue of His decrees alone. Pleasure and pain, reward and punishment, on this interpretation, are the means by which we are led to know God’s nature, which, once known, motivates obedience to His laws. This mechanism supports Locke’s claim that real happiness is to be found in the perfection of our intellectual nature—in embarking on the search for knowledge of God, we embark on the intellectual journey that will lead to the kind of knowledge that brings permanent pleasure. This at least suggests that the knowledge of God has the happy double-effect of leading to both more stable happiness and the understanding that God is to be obeyed in virtue of His divine will alone.
But given that all human beings experience pain and pleasure, Locke needs to explain how it is that certain people are virtuous, having followed the experience of dissatisfaction to arrive at the knowledge of God, and other people are vicious, who seek pleasure and avoid pain for no reason other than their own hedonic sensations.