Watching a Twitter conversation unfold today, I was reminded for the hundredth time that debates about same-sex marriage tend to be so fruitless because SSM doesn’t really mark the point of disagreement. There are already preceding disagreements about what marriage itself is and is for, but even those are not foundational. People disagree about all these things because they disagree about what human flourishing (eudaimonia) is, about what kind of life is generally speaking best for human beings to live.

For instance, if you’re a libertarian you will ipso facto have a high regard for personal autonomy and will be willing to pay a pretty heavy price in other goods — family stability, for instance — in order to preserve and extend autonomy. That won’t be your only value, but you wouldn’t be a libertarian if it weren’t near the top of your list. And the emphasis on autonomy will have a major influence on what you think marriage fundamentally is and on the rules you’ll be willing to accept for getting into and out of it.

Conversely, if you’re a traditionalist Christian, who thinks of the summum bonum as life lived in imitation of Christ and in communion with God, all pursued within a uniquely constituted Christian community, then your account of flourishing will derive from that. And you will think of marriage as an image of the union between Christ and his Church, and child-bearing in terms of raising vibrant members of the body of Christ. (Marriage and child-bearing won’t be wholly constituted by those goods, but they will be irreplaceably foundational to your picture of marriage and child-rearing.)

This isn’t the whole story, of course. In practice things get more complicated: for instance, a traditionalist Christian might become a contingent libertarian because he or she thinks that in a libertarian society the Church is most likely to be left alone to do its work and to be what it’s called to be. (I.e., In the secular realm y’all can call marriage anything you want as long as you leave us Christians alone to define it as we wish within the Church.) But if you’re arguing about SSM, or any other deeply controversial moral issue, and you’re wondering how someone else can read the situation so differently than you do, then the answer is likely to be found by digging into one another’s deepest beliefs about what makes for a really good human life. In light of those beliefs many other beliefs can become far more understandable.