A union where society recognizes that a partner can share or exercise some of the other’s rights in perpetuity.

From 1996 to 2015, public opinion on same-sex marriage in the U.S. changed dramatically, with 27% of the population supporting it in ’96 and 60% supporting it in 2016 . That’s an amazingly quick change of opinion for a large amount of a people in a short time. Many other social issues have taken generations to see that kind of change in opinion.

The interesting thing here is that the idea of marriage is incredibly old—it’s been around for thousands of years. And yet, within two decades, millions of people had changed their minds about how it works. Did they suddenly start using a new definition of marriage? Or maybe they’d never really thought about the definition of marriage before, and once they did, they changed their minds?

Our national change in opinion is even more surprising given how marriages have changed over the last hundreds and thousands of years. Is it possible to come up with a single definition that could cover all of them?

Looking at the definitions of marriages we might see today, we can determine that they’re not very reliable:

• the state of being united as spouses in a consensual and contractual relationship recognized by law

• any of the diverse forms of interpersonal union established in various parts of the world to form a familial bond that is recognized legally, religiously, or socially, granting the participating partners mutual conjugal rights and responsibilities

• the legally or formally recognized union of two people as partners in a personal relationship

The first definition doesn’t work because it relies on an understanding of the word “spouse,” which is defined as someone who’s married, so the logic here is circular. Without “spouse,” marriage is just a contractual relationship recognized by the law, and this includes any agreement two people make by signing a contract.

The second definition relies on the way marriages are already recognized, so it doesn’t help us if we’re considering changing marriage laws or norms. Someone arguing against a change in marriage law could always say, “But marriage is what’s recognized now legally or religiously, so we can’t change.” That definition also doesn’t cover anything that used to be called a marriage but wouldn’t be recognized today as the same. So, what should we call those “old” marriages instead?

The last one is probably the best of these options, because it uses “personal relationship” as the defining characteristic of the recognized union. However, I still see some problems with it. It limits marriage to just two people, which isn’t always true depending on the time or place we’re talking about. Also, personal relationships aren’t just marriages. If a single parent adopts a child, that’s a legally recognized union of two people. So we want something that will recognize certain kinds of personal unions and not others and that doesn’t rely on a circular definition.

A possible way to define a difficult term like this is to use a cluster or group of concepts. For example, with marriage, we might use concepts like love, living together, children, commitment, wedding rings, a ceremony, and tax breaks. Then, we can define a marriage as something that shares many of those concepts but not necessarily all of them.

However, no single concept would have to apply in every case. A cluster definition makes it very difficult to actually use the definition, especially to exclude a case where it shouldn’t apply—for example, in the case of a single parent adopting a child. That relationship doesn’t include wedding rings or a ceremony, but it checks off the rest of the boxes. It would seem to count as a marriage just as much as a couple without children who were living together would because it still hits most of the cluster terms. If we try to fix this problem by adding more terms, it might work, but then we’d probably dismiss examples of marriages that we want to include where only a couple of the terms apply.

On the one hand, it makes sense that marriage is difficult to define; it’s a complex concept built on thousands of years of history that has also changed substantially over that time. On the other hand, it’s an idea that people can recognize easily, which means we should probably be able to find some underlying logic that consistently ties all those examples together. What are the universal features that let us make those distinctions?

One common element to marriage is that it must be acknowledged by other people; whether it’s legal or religious or just “socially” recognized, it’s more than just two people acting differently or holding a specific belief. Imagine the last two people alive on Earth, if they wanted to be married, would they be? They could certainly have their own ceremony, but is two people making promises to each other a marriage? Maybe, if they viewed the ceremony as religious that would seem like a marriage, however that would still imply they thought there was third party blessing the union. There would be “someone else,” a deity in this case, that they think would recognize the marriage when they carried out the ceremony. The recognition by someone else is a critical part of the definition.

The other idea that stands out to me is that a marriage can grant the “partners mutual conjugal rights and responsibilities.” That’s a little vague, and “conjugal” only means something having to do with marriage. But is there something about the idea of rights and responsibilities in a marriage that can help us define it?

“Responsibilities” is probably too restricting, because it implies something that’s required. For example, when people get married, they often have many characteristics; they’re in love, they live together, they might have children and share property—but none of that is required, there’s no consistent set of behaviors that are always present in all marriages. A couple could live apart, with their own finances, not have children, and not even be in love, and we’d still recognize that they’re married. They wouldn’t be responsible for acting in any particular way, but we’d still recognize that they’re married because they would still have rights they could exercise. If one were in the hospital, the other could visit and make medical decisions on behalf of the spouse. Of course this doesn’t happen often, and when it does a spouse isn’t legally required to visit. They don’t have a responsibility and so there’s no behavior that we would always observe, but they do have rights because they’re married that we recognize even if they never actually exercise them. This couple, hypothetically living apart and on their own finances, without children, and without being in love, would still have rights because they’re married. Adding the idea of sharing rights then gives us this for a definition:

A union where society recognizes that a partner can share or exercise some of the other’s rights in perpetuity.

Does that work? In any society, there is some kind of formal or informal government or at least some set of laws or norms that are followed. And these laws include, either implicitly or explicitly, individual rights, rights that are either granted by a government or society to individuals or that are considered natural and inalienable. We all have rights that we can choose to exercise for ourselves but that we don’t have to. Marriage is the unique situation where someone else can now choose to exercise some of your rights as well.

For example, unless there’s a legal agreement otherwise, a spouse always has the right to shared property. Historically, the way these rights were shared wasn’t always fair. An ancient king undoubtedly had more rights to the property of his many wives than they had to his property, but there was an implicit recognition of shared rights.

The history of shared property is an interesting case because, almost universally, if property is going to change or have joint ownership, there is a transaction or a contract or some kind of agreement. But in a marriage, it’s the exact opposite. When the marriage is recognized, the change in rights to property happens by default. In different societies and at different points in history, the rights that one spouse could exercise or share aren’t always the same, but the fact that there are some shared rights is universal. It’s something that’s so consistent in every situation I can think of that, if there’s a union that doesn’t share this characteristic, we should really question whether we should even call it a marriage.

In countries like the U.S,, it’s easy to find examples of rights that spouses share, and one great example is the right against self-incrimination, which means you can’t be compelled to testify against yourself or your spouse in court. This right for the individual is clearly stated in the 5th Amendment of the U.S. Constitution. You can choose to testify in a trial against yourself, but you don’t have to. Spousal privilege is unique because, while it wasn’t written out in the 5th Amendment, it parallels the individual right completely. When you get married, you can now share in your spouse’s right to not incriminate him or herself, which really highlights the difference between marriage and other kinds of agreements between two people. You can sign a contract with someone to share property, and that’s just between the two of you. But it’s not possible to sign a contract saying your business partner doesn’t have to testify against you. That involves a right that the state or society has to recognize, which, in the case of self-incrimination, is only possible through marriage.

Why do we let people share rights like this? Because it makes the functioning of society easier and more stable for everyone. Some people want to live together, share property, and start a family no matter what legal status they have, and all those actions are easier if we recognize the union and allow those people to share a few important rights. It wouldn’t make sense to allow it with just anyone; it would get incredibly difficult to track if it were an easy union to enter in to and out of. But if someone is willing to make a commitment, the benefit of stability to society is worth it. Legal issues are easier to resolve when healthcare and family and property can be shared, when people have a safety net and someone else to make choices about their rights when they can’t in these important areas.

Looking at it this way also makes it clearer why people were able to change their minds about same-sex marriage relatively quickly. A lot of people probably thought that “one man and one woman” was part of the definition of marriage. But when we look at the idea closer, the sharing of rights is what’s really important and genders don’t really matter, so any people who want to get married should be able to share their rights regardless of gender. It also makes it clear why ridiculous claims such as “If we allow same-sex marriage, then we’d have to let people marry their toaster” don’t hold water—toasters don’t have and can’t exercise rights, so clearly they can’t get married

There are also practices that used to be common with marriages that aren’t recognized anymore, for good reason. A marriage for the purpose of combining two families’ properties doesn’t make sense now, because it’s a lot easier these days to accomplish the same thing with a simple contract. Throughout history, people didn’t always have equal rights, and that meant that when they got married, the way rights were shared wasn’t equal. It was still marriage, still a sharing of rights, it’s just that the underlying rights that were being brought to the union weren’t equal. Today, if people have equal rights, then the rights shared in marriage should be equal as well. Marriage seems more equal today, but it’s not necessarily that marriage has changed—it’s that people’s rights have changed.

A good definition doesn’t only let us see how or why we use certain words, but it also reveals underlying trends and logic that weren’t obvious before. More importantly, it allows us to make choices that are easy to defend and explain. In this case, I don’t think that the dramatic shift in opinion on same-sex marriage happened because people found a new definition like this one, that arguably works better than what we had and changed their minds immediately. But I do think many people recognized that the definitions we had been using weren’t working and that there had to be a better, more consistent one—because they wanted to recognize same-sex marriage even if they didn’t know what a consistent definition of marriage would be. Now, with this definition, we can see the logic that was always there that drove people to want to change their opinion.