What is wrong with incest (or anything else for that matter)?

Somebody asked on a forum a while back why incest was wrong. Usually when Christians are asked why certain moral claims they make are true, especially when it comes to sexual ethics, they try to answer them by appealing to broader moral principles the other person hopefully will agree with. The typical response to why incest is wrong is because it leads to birth defects (and the unspoken premise is that it's wrong to do what leads to birth defects). There are weaknesses to these kinds of responses, though. Some of these negative consequences can be ameliorated, but they are no less wrong. Incest would be wrong even if it happened between two people incapable of reproducing.

So I wanted to argue in a different way. I wanted to argue that there must be some moral principles that don't require justification by appeal to broader moral principles. After all, the hidden assumption behind questions about morality is that there always is a broader moral reasons for various moral claims, and the assumption is that if we can't justify a moral principle by appeal to another moral principle, then we have no justification for accepting the moral principle at all. I wanted to challenge that.

Also, this question was not asked specifically of Christians. I wanted to avoid making the claim that what made incest wrong was that God forbids it because that would derail the conversation and turn it into an issue of whether there was a God, whether God is necessary for morality, wheteher God was sufficient for morality, whether God does, in fact, forbid incest and if so, why. I wanted to avoid all that, so I made an epistemological argument for why we should accept some moral principles even if we can't justify them by appeal to broader moral principles. Here's what I said.

There's a variation on your third argument called "moral intuitionism" or something like that. The idea is that while most of our specific moral conclusions can be justified on the basis of inferring them from prior general moral principles, that can't be the case with everything moral that we know. Take this argument for example.

• It's wrong to harm people needlessly.
• Punching a stranger in the face harms them needlessly.
• Therefore, it's wrong to punch a stranger in the face.

This is an argument for the immorality of punching a stranger's face. That moral conclusion is inferred from a prior general moral principle--the wrongness of harming people needlessly.

That is how we derive most of our moral principles, but it can't be the case that all of our moral principles are derived this way because that would lead to an infinite regress. While the above argument explains how we know that it's wrong to punch a stranger in the face, it doesn't tell us why it's wrong to cause people harm needlessly. That first moral premise still needs some kind of justification.

You can justify that first moral premise in one of two ways. You can either do it using another syllogism with that premise as the conclusion and some other moral premise from which you infer the conclusion, or you can say the moral premise is known by intuition.

Intuition, in this context doesn't carry the same meaning as the colloquial use of the word. Usually when people say "intuition," they're talking about a subjective hunch about things. That's what people mean when, for example, they say "women's intuition." But that's not what intuition means in this context.

In this context, intuitions means immediate knowledge upon reflexion. There are some things we know simply because our brains are hardwired in such a way as to be able to apprehend them merely by thinking about them.

For example, you can know that two plus two is four just by closing your eyes and thinking about it. This kind of knowledge is different than knowledge through experience. You don't have to go out and test to see if two and two make four. Imagine discovering, by accident, that if you take two orange, and you find two more oranges, that lo and behold, together they make four oranges. And then imagine thinking to yourself, "Well, I know it works with oranges, but how do I know it also works with bananas? I better go test that, too." That may be how we discover how physical laws work, but addition is different. You can merely close your eyes and think about it and realize, through intuition, that it will hold for all things, anywhere in the universe.

Well, moral intuition is kind of like that. If we had to prove each moral principle by appealing to some prior moral principle, that would lead to an infinite regress, in which case we couldn't know any moral principle because we could not complete an infinite number of inferences or know an infinite number of moral propositions. So if we have any moral knowledge at all, then it must be ultimately grounded in one or moral moral principles that we can know without having to infer them from prior moral principles.

So if we grant that there are at least some moral principles that we know to be true, then it's necessarily the case that there are some moral principles we can know by intuition. Without intuitive knowledge, no knowledge of anything would be possible because it's the only way to avoid an epistemological infinite regress.

So it could be that we just know intuitively that incest is wrong. And that does seem to be the case because people have an easier time coming to the conclusion that incest is wrong than they do explaining how they know it's wrong. Whenever somebody is questioned with, "How do you know?" they always try to justify their knowledge with reasons. But we see people frequently claiming to know things while finding it difficult to put their finger on how they know it. The knowledge is more obvious to them than whatever reasons they cooked up to justify it. That means the knowledge comes first, and the real reason they know it is through intuition.

This is why people believe that their senses are giving them true information about an external world that actually exists. Since things would appear exactly the same whether the external world was real or whether you were just a brain in a vat, there's no observation you could make to adjudicate between thinking the world is real and thinking it's an illusion. But that doesn't mean the general population is all 50/50 on whether the world is real or not. We all instinctively believe the world is real until philosophy talks us out of it. So our knowledge of the external world is intuitive.

If you deny knowledge by intuition, then you have no justification for believing in the external world. Nor do you have justification for believing in the past since it's possible we were created five minutes ago complete with false memories. More than that, without knowledge by intuition, it would be impossible to know anything at all because without intuition, every line of reasoning would lead to an infinite regress.

This doesn't mean that intuition is infallible. There are some things, like two plus two make four, that we can know with certainty, but there are other things, like the external world, that we cannot know with certainty. So the most parsimonious rule of thumb we should use to think things are more or less the way they appear to be unless we have good reason to think otherwise. The default position we should take is that there's an external world because it looks like there's an external world, and we have no good reason to think otherwise.

The same thing applies to incest. It seems to almost all of us that incest is wrong, even if we can't put our finger on why it's wrong. So we are justified in believing it's wrong in the absence of any good reason to think otherwise.