All arguments attempting to ground ethics in nature or human nature are perfectly unreasonable, because such arguments presume an answer to the question in dispute.
The question is whether natural duties can exist without a supernatural sovereign authority defining and enforcing that duty.
Natural duties include those things we ought to do, regardless of current inclination, self-interest, or consent. Natural duties are sometimes called involuntary duties.
Covenantal duties include those things we ought to do, regardless of inclination, because they serve the self-interest of family, clan, nation, or civilization, whose benefits to us impose reciprocal obligations.
A covenant can be repudiated if the civilization or nation or clan or family has betrayed the foundational obligations on which it is based, and become destructive or self-destructive.
Covenantal duties are sometimes called enlightened self-interest, or called a social contract, albeit obviously no explicit contractual terms are written anywhere, and no individual consent is asked or needed.
Contractual duties include those things we ought to do, regardless of inclination or self-interest, because at one time one consented. A contract can be repudiated if the other side is in breach. Covenantal duties are sometimes called voluntary duties.
Generally, and argument grounding ethics in nature fall into several camps: one can argue that ethics is grounded in group survival; is grounded in self interest rightly understood; or is grounded in self-evident conscience.
It is unreasonable to argue that natural Darwinian selection rewards cooperative group behavior, and so instills in the instincts and passions of man a desire to foster peaceful amity within the bloodline, including family, clan, tribe, and race, so as to maintain the bloodline and see it prosper: and the rules formalizing racial maintenance and prosperity are identified as ethical duties.
This argument refutes itself under its own terms, since it merely describes "duty" as being precisely what it is not, namely, a natural inclination. This is, indeed, the naturalist fallacy. Once cannot deduce an "ought" from an "is."
Describing the speculative historical origins of the inclination offers no normative argument one way or the other on the question of when and if and why one should obey this inclination.
One might have a natural inclination to eat when hungry, and see this is necessary for health, or an inclination to gluttony, which degrades the health. Again, one might look with rapt erotic longing to one's lawfully wedded wife, or for one's sister or brother or child or family pet. The mere fact that the inclination exists does not necessitate that a duty to obey it likewise exists, rather than a duty to resist it.
In any case, proposing a Darwinian ethical system requires affirming the extermination (or, at least, the castration) of other races and tribes with whom one shares no bloodline, and require exposing the weak to the necessary culling of natural selection, lest the race degenerate.
Likewise, It is unreasonable to argue that that human reason allows us to grasp that short-term pleasures often lead to long-term regrets, and that therefor prudence is necessary to distinguish good pleasures from regrettable ones, and that temperance is necessary to govern the pleasure seeking impulse.
For the same reason, courage and patience are needed to govern the self-preservation impulse when fear or frustration may well lead to regrettable action.
Finally, when taking the actions of one's fellows into account, who will naturally retaliate against aggression aggressively, but will naturally cooperate with cooperation for mutual gain, justice is needed to see to it that cooperation is rewarded, and aggression deterred.
This argument defines the pursuit of one's impulses so as to take long term effects into current account, or the likely reactions of one's fellows, as self interest rightly understood, also called enlightened self interest.
Self interest rightly understood requires the virtues of prudence, temperance, fortitude, and justice be promoted and followed, as prudent instruments of one's own pleasure and long-term prosperity, that is, for the sake of one's own self-interest, even if not for their own sake.
This argument assumes its own conclusion. If one has no duty to be enlightened, nor to serve one's own self-interest, that is to say, to be prudent when pursuing one's impulses, the mere fact that it would be more prudent to take the long term into account, or the actions of other men, does not itself impose a duty to be prudent.
In any case, merely learning to quell the subjective sense of regret following the various hangovers or addictions, bastards or infanticides, venereal diseases, and anarchy which imprudent self-indulgence in vice and crime provoke would be no less an operation of the self-control than the cultivation of virtue needed to avoid such vice and crime.
By this logic, the prodigal son who wastes his livelihood in wine, woman, and song, provided he schools himself to be content afterward to die prematurely in a pigsty, stealing swill from hogs to eat, dying bravely and without regret, has no duty to use his virtue, such as it is, to seek any other outcome.
Don Giovanni experiences no iota of regret for his action, but is bravely defiant of his infernal fate to the last. Yet all men of goodwill regard him as not the same as a hero like Horatio who dies without regret facing fearful odds for the ashes of his father and the altars of his gods.
But without a natural duty to use one's virtues for good rather than evil ends, Don Giovanni and Horatio are twins. Merely the object for which he is bravely willing to sacrifice his life to purchase changes: a life of deception and cruelty, rape, and murder in the case of Don Giovanni, versus a life of virtue and heroic valor in the case of Horatio.
Hence prudence and fortitude the other virtues of self-control are insufficient, by themselves, to serve as a ground of natural duty.
Likewise, it is unreasonable to argue that the natural duties are self-evident, appearing as intuitive maxims in the soul of every civilized man, and appearing as inchoate percepts even in the soul of barbarians.
Again this argument assumes its own conclusion: even granting that something in the human soul naturally imparts an undeniable knowledge that to treat others as one would not oneself wish to be treated does not, in and of itself, impose a duty to act on this knowledge. The argument assumes that one has a natural duty to abide by the Golden Rule, or other maxims of moral principle, the moment they are apprehended by the reason, and affirmed by the conscience.
But unless there is a natural duty to obey the conscience, even if one found oneself in a world where the conscience reported self-evident and intuitively-obvious maxims of behavior, it is neither evident nor obvious that one ought to do the self-evident good ethical act, particularly when one is not so inclined by immediate inclination or by self interest rightly understood.
Let us turn away from these dead ends, and see where a reasonable way leads.
It is perfectly reasonable to point out that contractual duties are grounded on a natural duty to keep oaths.
Obviously a creature under no duty to keep his oath cannot be asked to make "an oathkeeper's contract" saying he will keep his oath thereafter, since, by hypothesis, he has no natural duty to honor his oaths to keep contractual duties.
It is also perfectly reasonable to say that, if there is no natural duty to keep one's oaths, that the reciprocal oaths of brotherhood and fealty and allegiance necessary to maintain discipline, organize war bands, erect judges and kings and seat parliaments and senates would not and could not exist, except, perhaps of matters of arbitrary personal preference. It is also perfectly reasonable to say that, if there is no natural duty to keep one's oaths, civilization fails.
And, again, it is reasonable to say that if civilization fails, anarchy erupts, and death and misery reign.
One's own natural inclination for pleasure, the desire for fellowship in peace and loyal brotherhood in war, the natural affection of family and clan and tribe, all incline a man to have an appetite and passion for civilization, and make the maintenance of civil concord in his self interest, even if this involves burdens of abiding by arbitrary rules of courtesy, bowing to often unjust laws, paying often excessive taxes, and being drafted to die for one's nation.
But there reason falls silent. All this is to no purpose. If there is no natural duty to serve one's self-interest, a man need not serve his self interest: he has no duty to avoid suicidal folly or self-destructive pleasures found in alehouse, opium dens, or cathouses.
If there is no natural duty to be civil and civilized, keeping as many concubines as unchecked lust can crave, and unchecked tyranny can abduct, is merely a matter of personal preference and practical calculation. Conquest, torture, murder, treason, and Machiavellian intrigue become also matters of practical calculation: what one can reasonably expect to achieve considering the costs and benefits of rapine and plunder against likely the chance of detection and the degree of retaliation.
A wiser Machiavellian, perhaps a Philosopher-King, it is true, in planning for matters further in the future, or embracing a fellowship of wider scope, could indeed conclude that teaching the public to abide by what seem to be objective moral standards, out of fear and respect for supernatural judgement which can never be important to punish nor reward, nor be deceived nor be eluded, just as one's own conscience cannot be eluded, and which has no boundary on its jurisdiction, would have many benefits in maintaining social cohesion, and obedience to laws even when enforcement by mortal men is impractical or impossible.
Morality becomes an opiate of the masses, a noble lie told by the Philosopher King to keep the ignorant underlings in check, perhaps for their own good, or the general good of society, or perhaps not.
But in such a scenario, the Philosopher King needs must deceive his Augustus and heir, his officers and ministers, so that they will also abide by laws he himself, with no more than his own two hand, has no ability to punish them for breaking. After his death into the oblivion of no afterlife, the edifice of pragmatic falsehood will continue for as many generations as long as all parties continue in the mutual social self deception. Which, since this mutual social self deception was designed to serve the long term self interest of society, has good reason to be maintained by that society, as long as the design remains sound.
Until an up and coming Philosopher Prince sees no lawful way to seize the throne, but he is philosophical enough to deduce that the noble lie is a lie, and imposes no duty on him to obey it, even if it will once against be in his self-interest to impose it on his gullible subjects. By its own logic, the noble lie has no moral authority over those who see it as a lie.
It might be convenient, as a practical matter, for a time, but unless there is a natural duty to uphold convenient lies, the noble lie cannot be binding on the conscience of one not inclined to believe nor promote it.
More to the point, a down and out Philosopher Anarchist, who has no love for the society, no gratitude for its benefits, and seeks only to wound a society he hates, once he convinces himself the noble lie is ignoble, will erect in its place the ignoble lie, namely, that there is no objective moral standard, and supernatural judge and lord to define and enforce it. Instead there is human will, or human history, which drives progress upward to utopia. Utopia is so fine and perfect that any crime whatsoever required to achieve this goal are justified, nay, obligatory. This permits every enormity imaginable, and some to horrible ever to have been imagined beforehand, to be committed by perfectly diabolical sociopaths garbed and masked in the sanctity of self-righteousness.
Under the ignoble lie, not only is the most blood-drenched Holocaust, Holodomor, gulag and deathcamp justified and obligatory, lesser crimes of mass extortion, mass theft, mass murder, mass rape, mob actions, riots, arsons, injustices, abuse of law, abuse of trust, treason, deception, destruction, and every trifle of discourtesy, blasphemy, and every sign of hatred and contempt down to the smallest gesture of insulting an assassination victim, it is the epitome of progress, enlightenment, and self-righteousness. Men must become anti-human; they must love death more than life; madness more than reason; they must welcome barbarism, anarchy, chaos, death.
All that the Philosophical Anarchist has done in this case is take the logic of the Philosopher King, and applied it consistently and honestly to all aspects of human life. If the noble lie is needed for civilization, then honesty is barbarism. If the noble lie is needed for life, the honesty is death. If the idea of objective morality is a noble lie, then honesty itself is immoral.
The endless misery, meaninglessness, madness and bloody horror as imagined in the dystopia of Big Brother and Brave New World, as well as the pure horror as is seen in Soviet gulags, Nazi death camps, Red Chinese torture chambers, or American Faculty lounges, where all speech and reason withers and dies, and all humanity is abolished, is the outcome of ethical naturalism.
Such is the only and inevitable logical outcome of any attempt to ground natural duty on anything other than a supernatural ground.
In sum, either natural duties exist, or do not. A natural duty is what one ought to do, regardless of inclination or self interest or consent. Duties based on self interest are covenantal. Duties based on consent are contractual. These cannot exist unless natural duties, which are non-consensual and non self-interested exist.
Natural duty cannot be manmade. Whatever duty is manmade, man can unmake and excuse when as it suits him. Nor can a duty arise from merely from pleasure-seeking, prudence, or man's own conscience.
Therefore natural duty is non-manmade, hence not to be abridged nor excused by man. This logically requires natural duty to be objective, ageless, ever-present, with no restriction on jurisdiction, and no rational creature immune or except. Nothing in nature is timeless and ever-present. Therefore natural duties are supernatural. Since unenforced duties are merely suggestions, a supernatural lawgiver, judge and lord is necessary for natural duties to exist. If, as has been shown, such duties are ever-present and timeless, they exist wherever anything that exists is in existence. Hence the judge and lord to enforce these duties is and must be a necessary being, not a contingent being. And this all men know to be God.
