This summer one of my sisters died. As much as we all miss her, none of us, including her own children, feels her absence as much as does her husband.

He doesn’t just miss her. Half his life is gone. That’s where he is different from her kids—they lost their mum, but still have a whole self left. He wakes up mornings, walks through days, and goes about the business of raising family and crops with some of his own body missing.

That’s no romantic exaggeration, as everyone who knew them knows. They were married, husband and wife for 34 years, and everything about them and their relationship suggested that what was between them was rare.

“And the two shall be one flesh!” That they were, just as the second page of the Bible describes it. Both had left their own families and a lot of other things to cling to each other, to be one flesh. When a man and a woman love each other in that way, truly in that way, each dies and something new, some third thing, is born.

In my sister and brother-in-law’s relationship, you saw this third thing, human love gone consummate, grown sacramental.

Small wonder, that my brother-in-law now feels only half alive. For a while he had an ally, a co-conspirator, against the most primal of all loneliness, the one that God himself damned at the origins of history: “It is not good for the man to be alone!”

For a time, he was not alone. He was married—married in a way that is worth reflecting upon:

What makes a great marriage? What made my sister and her husband “one flesh” in a way that is so often denied the rest of us? What really marries one person to another?

There are all kinds of answers to that question and, given a culture that constellates so many of its feelings about love around sex and romantic obsession, what was true in their case is normally not what comes first to mind.

They weren’t Romeo and Juliet. Their’s was not the stuff of Hollywood romances and Iris Murdoch novels. It had such a quietness to it, a gentleness, softness and chastity, that it contained nothing of those exaggerated forms that make for great art—and often for tumult, heartbreak and infidelity in real life.

Nothing between them garbled life. Their relationship was, for the most part, too ordinary to notice. They didn’t often get the chance to look at each other over crystal wine glasses under romantic lighting, though they yearned for that. They had to catch each other’s eye more domestically.

For whole years at a stretch, over dirty diapers and dirty dishes, in a house packed with kids, they would meet each other’s eyes and both would know that they were home: “At last, bone from my bone, flesh from flesh.”

They knew what consummation meant. For 34 years they had only to look at each other to not be alone.

But what makes this? What needs to be there for someone to look at another and feel that other as bone from my bone, flesh from my flesh, kindred spirit? In today’s terminology, what makes someone a soulmate? What do you need to experience with another person to overcome that exile of heart?

Someone looking at my sister and brother-in­law might, more superficially, have seen some obvious things: deep mutual respect, a gentleness between them, uncompromising fidelity to each other, harmony of thought and feelings on most things important, regular prayer together and a complete trust of each other. Those things are the heart of a marriage.

But, in the end, these were, in their case, symptoms really. What connected them, made for bone of my bone, for the harmony, respect, fidelity and gentleness was something deeper. They had moral affinity. Long before, and concomitant with, sleeping with each other physically, they slept with each other morally.

What’s meant by this curious phrase? Each of us has a place inside where we feel most deeply about the right and wrong of things and where what is most precious to us is cherished and guarded.

It is when this place is attacked that we feel most violated. It is also the place where, in the end, we feel most alone. More deeply than we long for a sexual partner, we long for someone to sleep with us here.

My sister and brother-in-law found this in each other. They were moral lovers. They found, touched and protected each others’ souls. Everything that was deepest and most precious in each of them was understood, cherished and safe when the other was around.

It made for a great marriage—one flesh, true consummation, all predicated on a great trust and a great chastity. This is a secret worth knowing.