From the early essays on the obligatory force of natural law to the careful revisions of later editions of the Essay, Locke continually displayed an intense interest in problems of moral philosophy. The proper aim of human knowledge, he supposed, lies not in the satisfaction of attaining abstract speculative truth, but rather in its application to practical conduct, upon which our happiness in this world and the next ultimately depends. [King, p. 86-88] Ethical knowledge is a variety of what he called
Praktikh, The Skill of Right applying our own Powers and Actions, for the Attainment of Things good and useful. The most considerable under this Head, is Ethicks, which is the seeking out those Rules, and Measures of humane Actions, which lead to Happiness, and the Means to practise them. The end of this is not bare Speculation, and the Knowledge of Truth; but Right, and a Conduct suitable to it. [Essay IV xxi 3]Since our cognitive faculties are best suited for pursuing that knowledge of ourselves and God that is most likely to lead us toward the dutiful conduct by means of which we may secure eternal happiness, Locke argued, morality is the most vital aspect of study for all human agents. [Essay II xii 11] In what ways do human faculties establish the foundations of moral knowledge?
Given their great importance for human life, practical principles would be among the best candidates for special status as innately provided by a benevolent creator; but, of course, Locke held that there is no innate human knowledge. Lists of purportedly innate practical principleslike the ones noted by Lord Herbertare in fact neither universally accepted nor reliably productive of correct conduct. The open, remorseless disavowal of moral principles in various cultures, along with the open question for their justification, is ample evidence that they are not truly innate. [Essay I iii 11-19] Only the desires to achieve happiness and to avoid misery are both genuinely universal among human agents and practically effective in guiding their conduct, Locke argued, so it is only the natural tendency to seek pleasure and avoid pain that might reasonably be held to be innate. To the extent that appropriate patterns of human conduct are found to be in widespread conformity with morality, he supposed, it is only in virtue of a providential association of moral rectitude with more short-sighted perception of personal and public welfare. [Essay I iii 3-6] Apart from these general inclinations, he believed, nothing about human morality is universally acknowledged.
Nevertheless, Locke was no moral relativist. Human moral discourse is subject to the kind of perfect precision that should yield the possibility of demonstrable truth. In principle, moral termswhich describe the varieties of human action and delimit the degrees of their rectitude in relation to moral laware all perfectly definable, since each signifies a mixed mode whose determinate content is secured by its manufacture in the mind. [Essay III xi 15-17] Although intellectual laziness, malicious arrogance, and culpable self-interest often render moral discourse problematic, Locke believed that careful, dispassionate attention to the complex ideas involved should produce demonstrable moral reasoning.
The mixed modes of human action are complex ideas derived from recombination of the simple ideas of thinking, moving, and power, so all the vocabulary of "Divinity, Ethicks, Law, and Politics" can be derived ultimately from our simple ideas of sensation and reflection. [Essay II xxii 10, 12] But notice that these ideas, and the words that signify them, can be fully formed in the mind independently of their actual instantiation. We can know what sacrilege and resurrection would be without experiencing their occurrence; we choose to define single words for "murder" and "stabbing" but not for other ideas equally familiar in experience; but we form abstract ideas of those human actions to which we most commonly refer, whether or not they frequently occur. [Essay III v 5-7] Thus, on Locke's view, such moral ideas as those of obligation, drunkenness, or lying, are formed by combining simple ideas from the mental and physical aspects of human nature without ever supposing that anything conforming to the new composite has ever existed. [Essay II xxii 1] This detachment from questions of real existence, Locke believed, is crucial for establishing the demonstrable status of human morality.
The moral rectitude of actions of a particular sort, Locke held, is wholly constituted by the demonstrable relation between our clear ideas of such actions and the equally clear conception of the moral law. Indeed, this relation is often so obviousas, for example, in the cases of "murder" and "theft"that the moral condemnation comes easily to be included as a part of our complex idea of the action itself. [Essay II xxviii 14-16] Because both my contemplated action and the moral rule can be abstractly conceived as mixed modes, the applicability of this rule to that action can be determined with perfect certainty. It is a further question whether or not the moral rule itself is demonstrably true.
Locke believed that it often is. To be sure, reliance upon an axiomatic deduction of morality from a fixed set of putatively indubitable first principles would be neither effective nor intellectually sound. [Essay IV xii 4-5] Nevertheless, demonstration is possible in principle wherever we have clear ideas, and Locke was careful to emphasize that indubitable knowledge of relations does not presuppose perfect clarity with respect to the relata. We might know that one automobile is faster than another, for example, even if we had little information about the mechanical differences that produce this result. Our awareness of relations commonly rises to a level of certainty greater than our knowledge of the things among which they hold. [Essay II xxv 4-8]
What counts toward demonstrability, on Locke's view, is the possibility of perceiving intermediate links between our ideas. Since the mixed modes of human action and the concepts of moral rules are both abstract ideas that serve as their own archetypes, it follows that the relations between them are fully demonstrable. [Essay IV iv 7-9] In this respect, at least, morality is on an equal footing with mathematics.
Where there is no Property, there is no Injustice, is a Proposition as certain as any Demonstration in Euclid: For the Idea of Property, being a right to any thing; and the Idea to which the name Injusticeis given, being the Invasion or Violation of that right; it is evident, that these Ideas being thus established, and these Names annexed to them, I can as certainly know this Proposition to be true, as that a Triangle has three Angles equal to two right ones. Again, No Government allows absolute Liberty: The Idea of Government being the establishment of Society upon certain Rules or Laws, which require Conformity to them; and the Idea of absolute Liberty being for any one to do whatever he pleases; I am as capable of being certain of the Truth of this Proposition, as of any in Mathematicks. [Essay IV iii 18]The apparent advantage of mathematical over moral reasoning, Locke speculated, rests only on the relative ease with which we can represent mathematical relationships in perspicuous diagrams and the relative absence of partisan concerns. Were we to approach moral reasoning with the same degree of objectivity we commonly bring to mathematical thinking, he argued, we would achieve the same quality of demonstrable certainty about substantive moral truths. [Essay IV iii 19-20]
In general, Locke held that the mental comparisons comprising our ideas of relations are significant for the practical conduct of life. Natural relations among human beings, like the various degrees of blood-kinship, for example, commonly carry the presumption of some special obligation toward other members of our families. Instituted relations based on social agreement ground the governance of human societies, as we'll see in detail in a few weeks. But moral relations are most vital of all, so that the very descriptions of human action under our ideas of mixed modes commonly carry with them an implicit reference to the moral law under which they are commanded or proscribed. [Essay II xxviii 2-4] Moral valuation, on Locke's view, derives from the demonstrable connections that hold among the ideas of duty, law, legislator, and sanction. [Essay I iii 12-13] Since no moral law could determine human volition and thereby influence human actions practically without careful provision for punishment and reward as artificial consequences of disobedience and obedience, it follows that moral legislation must derive from legislation by intelligent beings with the power to enforce their dictates by appropriate moral sanctions. On this basis, Locke distinguished three basic types of moral law by reference to the legislative source of each: divine law, civil law, and the law of opinion or reputation. [Essay II xxviii 6-7]
The divine law arises from God's right as creator to dictate morality to all creatures of his own making, his wisdom and benevolence guiding them toward what is best, and his power to enforce this law by distributing in the hereafter punishments and rewards that are both infinite in extent and eternal in duration. Thus, Locke held that the resulting distinction between duty and sin is "the only true Touchstone of moral Rectitude," founded upon the ultimate happiness or misery attached by God to actions of particular sorts. [Essay II xxviii 8] Thus, Locke held that denial of God's existence, moral legislation, or control over eternal life can only be attributed to an irrational hope of escaping moral law and the divinely ordained consequences of sin, since no one who professes such outrageous opinions is observed to live a life in accord with the Golden Rule. [King, p. 90] In the second edition of the Essay, Locke carefully noted that the divine law may be known either through revelation or by the manifest light of natural law. In either case, he supposed, the divine law guides human conduct so obviously toward genuine happiness and away from profound misery that even public opinion and private interest commonly defer to its force: even those who behave badly themselves often praise or blame others by reference to the genuine criterion. [Essay II xxviii 8, 11]
The civil law derives its force from the legislation of a government to which its citizens have already consented. Since the commonwealth has been formed for the purpose of protecting "the Lives, Liberties, and Possessions" of its citizens, it has the power to take away any or all of these goods in punishment for the crimes of disobeying its rules of conduct. [Essay II xxviii 9] Although its penalties are more limited than the infinite sanctions of divine law, Locke supposed, their certainty and immediacy provide a secure basis for enforcement of the civil law.
Finally, the distinction between virtue and vice belongs only to the law of opinion or reputation and is sanctioned only by the praise or blame of others. Although public opinion always praises the virtuous, Locke noted, the standards of virtue and vice vary widely among different cultures, though some degree of conformity to the rational dicates of natural law is always to be expected. Although this level of moral law derives from a source no more significant than what other people happen to think, the threat of "Condemnation or Disgrace" from one's fellows is a powerful motivation for many human agents; few of us willingly invoke the disapprobation of others. [Essay II xxviii 10-12]
The emphasis on punishment and reward in these accounts of moral law draws attention to an important distinction between the grounds of moral obligation and the motivation for obeying it, both of which derive from the legislator. We are morally bound to obey because the creator has a right to command, but we are practically moved to obey because God has the power to punish us if we don't. The practical force of moralityits capacity to determine volition and influence actionderives from the punishments and rewards that secure our compliance. Thus, Locke argued, good and evil are nothing other than pleasure and pain; moral good and evil are just the pleasure and pain artificially annexed to obedience and disobediance by the decree of the powerful legislator.
[Essay II xxviii 5-6]
Moral motivation requires only that a rational agent consider the possibility of future pleasure or pain that will result from present actions, and Locke believed that the prospect of eternal happiness or misery ought therefore to weigh upon us at least as firmly as more short-term expectations.
[Essay II xxi 70]