The antiquity of Reason

Oscillon ( asks about this (quoting me) “A Modernist is stumped if you ask him why incest should be illegal: either reluctantly or eagerly he will sooner or later say it should not be illegal, because this is the logical outcome of the libertarian hedonism which he takes as an axiom. (The other option is that he will find some ad hoc excuse to forbid the practice, such as the question of genetic recessives.) “

Oscillon’s comment:

I’ve heard this a few times now. The problem I see with it as a method of argument is that you are asking the ‘modernist’ for reasons/logical argument but you are not holding the ‘conservative’ to the same standard. At least, I have not heard the logical/reason based argument for why it is illegal from you. Saying, “it should be so because it has been in the past” is weak. The same argument could have been made for retaining slavery or a host of other evils that were once considered normal or ‘conservative’. In this country’s recent past, segregation was the conservative position. You can point to some slim segment of the conservatives that did not agree but the great majority of conservatives (and the general public) thought segregation was just fine. In fact, the people who fought against it were branded radicals who would undermine society.
I know people old enough to have been on both sides of the segregation issue. They grew up when it was normal and they thought it was moral. They now realize with the same or greater conviction that it was immoral.

This is not an argument that incest is ok and to answer it so is a diversion. The point is that arguing a point by saying “we’ve always done it that way” is not sufficient.

My comment:
First, I am not sure I understand what you are asking. My argument boils down to a simple proposition:
1. If we assume that wrong consists only of violence or trespass done to another without his informed and adult consent, then we conclude incest is not wrong. (if S then P)
2. Incest is wrong. (not P)
3. Therefore we may not assume wrong consists only of violence or trespass done to another without his informed and adult consent. (therefore not S)
This is modes ponens.
Second, While you have asked me not to address the proposition that incest is wrong, I must point out the simple logic that, if it is wrong, the fact that it has been always held to be wrong by all men for all time is not  a coincidence. Hence, the antiquity of the proposition, while not proving the proposition, is not unrelated to our conversation. 

I must ask in that regard what, in your judgment, you hold to be the ground of morality? Until I know what you take to be the ground of morality, all I can say is that logic only can operate from the clear to the unclear, not from the unclear to the clear.
We have here a moral proposition that is as obvious and universal as any moral proposition can be: fathers should not couple with their daughters, sons with mothers, nor brothers with sisters. Compared to this, the proposition that wrong consists only of violent aggression against the rights of others, for example, is less clear.
This axiom of non-aggression may be true, but it is not self-evident, nor is it universally known. Indeed, were it proved true, I would call it a rare and late discovery of the moral progress of mankind.
But I can say what I think the ground of morality is. I think the human conscience gives us, by and large, correct and real information about the nature of reality around us, in much the same way an eye sees empirical objects, or the reason deduces mathematical objects. The conscience allows us to grasp moral objects.
Do you accept the proposition that reality is not arbitrary, and that human emotions are healthy, virtuous and correct when they are fit and proportionate to that reality, and otherwise they are unhealthy, vicious, incorrect?
By this I mean, for example, that it is fitting, not a matter of personal choice or opinion, for a child to feel filial love for his mother; it is fitting, not a matter of personal choice or opinion for a mother to feel maternal love for her child; it is fitting, not a matter of personal choice or opinion, for a father to feel paternal love toward his family, piety toward his ancestors, patriotism for his homeland, courage when facing his enemies, generosity toward his friends, justice toward all men.
You can I cannot discuss how the passions should be ordered until and unless we can agree that there is an objective order to the universe that renders some passions fit and proper and others unfit and improper toward their several objects. Is this an axiom you accept?
If you do accept this axiom, we can begin to speak about which moral principles are primary and intuitive and which are secondary and deductive. Some moral rules by their own force are obvious the conscience; others are deductions or extrapolations from rules of the first kind.
For example, the rule against fratricide is primary: brother should not kill his brother. The rule against murder strangers is a deduction, an expansion of this accepted principle to include more than brothers. It is a deduction that primitive people (or civilized people flirting with savagery) do not deduce: namely, that every stranger is someone’s child, all men are brothers.
If the rule against incest is primary, its antiquity is a strong argument for its legitimacy. We know this rule is moral because we have always known it. If, on the other hand, the rule against incest is secondary, its antiquity tells us nothing in particular. Our ancestors preyed on strangers and enslaved them because they failed to make the moral deduction about the brotherhood of man. In that case, the antiquity of the practice was merely a sign that the moral science of mankind had not advanced to the correct point.
So is the rule against incest a primary or secondary rule? If it is a secondary rule, what is the deduction of the moral sciences alleged to support this new rule, and how well does it fit in with the previous precedent of moral thinking?
Third, if you are asking me to distinguish the case of incest from the case of segregation or slavery, I need only point out what the argument on the other side is. Every real moral question consists of tension between two acknowledged moral principles that lead to different results. In the case of slavery, we have on the one hand the principle that says the weak captured in battle may be treated as chattel, since they would surely prefer that to being slaughtered; on the other, the principle says simple justice demands we treat these men as we would like to be treated, and restore them to their native liberty. (Naturally, there are other arguments to be made, but I am simplifying for the sake of an example). Justice is the higher ideal over the convenience of enslaving prisoners.
In the case of incest, the argument on the other side is that any man should be allowed to do whatever he will, when he will and as he will, provided only that he obtains the informed and adult consent of those his actions touch. This is not a moral principle at all, merely an expression of untrammeled and ungoverned passion. Indeed, it sounds at first blush like the mere opposite of a moral principle, since moral principles consist of encouraging the passions in cases where they are suitable, and discouraging them in cases where they are not suitable.
This so-called principle says a passion is not suitable when and only when your partner in the venture does not consent: but it says nothing about by what standard the partner should judge when she should or should not consent.
In other words, the proposition that a sister who consents is a proper object for copulation is not any more obvious than the proposition that the daughter of a stranger who does not consent is a proper object for violent seduction. Why should I or anyone have more respect for incestuous sex than for non-consensual sex?
Since, upon examination, no real argument can be imagined that incest is right, we can use our judgment to infer that incest is wrong.  
Forth, if you are asking me to accept the proposition that precedent does not control law, I must reject that proposition. To say we should do as we have always done is a sufficient reason for any man who thinks his ancestors were as wise and human as himself. The only times where the precedent of our forefathers should be over-ruled is a case where we have made progress from barbarism toward finer civilization: that is, where we can identify the specific point where their understanding of the moral hierarchy of the universe was mis-ordered, and we have corrected the order. This is possible if and only if there is a moral order to the universe.
If all is merely a matter of opinion and fashion, what we have is change, not progress: we drive on the right side of the road and they drove on the left. That is merely a change. We are citizens who vote for our leaders; they were subjects of a king. Their ancestors in turn were slaves of a tyrant, and their ancestors lived in primitive anarchy, the war of all against all. To move from anarchy to liberty is to gain continually what is desired of the art of politics: peace and wellbeing. To move from the lefthand driver to the righthand driver is not a gain of anything in particular.
But, once you accept that the concept “progress” presupposes the concept of an objective moral order to the universe, one cannot call it “progress” to abandon one of the core ideas of the moral order. It is not sufficient to say I should be allowed to marry my daughter because I love her, any more than it is sufficient to say I should be allowed to poison my son because I hate him.
Fifth, let us distinguish laws, which govern one man’s relation with his neighbor, with virtues, which govern the various balances and proportions of passions and appetites in a man’s soul.
If you say that morality consists only of law, and not of virtues, common sense will dismiss this proposition without further ado. As a matter of law, the idea that no man should trespass into the rights of others is sound as far as it goes. But from this, the libertarian conclusion that virtues are entirely private matters does not follow. Experience shows that if a man cannot govern his passions, he cannot govern his actions. No mother would raise a child lauding and rewarding his surrender to anger, and then punish him for acting on the anger. The action follows from the passion. One causes the other.
Logically, good laws cannot preserve a commonwealth made of people devoted to bad virtues, because it is their own internal virtues that cause or create their external behaviors. The laws will not be obeyed unless they are obeyed with the general spontaneous support of the majority. Strict laws will not make a corrupt people law-abiding. In theory a person can have respect for law while having contempt for morality; in reality, it never seems to work that way.
In this case, the zenith of corruption is the libertarian proposition that all men are free to disorder their own passions to any extravagant extreme, provided only they do not trespass into the rights of others. But, in reality, men without self-control and without virtue will not have the fortitude needed to resist the temptation to trespass on the rights of others, not after a lifetime of being taught that base self-indulgence any and every temptations is lawful, right, and proper.
I say again that no proposition is more obvious in moral reasoning than the law against incest. Correct me if I am wrong. Tell me something that is even more obvious, will be more clear to all men, or has so long a history so firmly rooted? Tell me anything else that every culture of mankind has agreed?
Again, I am not saying the rule against incest is obvious. I am merely saying nothing else is more obvious. If it is obvious, the observation that it has always been obvious is pertinent to the conversation. Just because we have always done so does not prove that we should continue, but it does hint that we are not dealing with merely a fad or fashion or arbitrary custom of our age.