In his book Bonfire of the Vanities, Tom Wolfe describes the women at an exclusive party in Manhattan. The first group, starved to near perfection, used fashion to compensate for the natural curves that they had denied their bodies. These were mostly the first wives and “women of a certain age.” Then he describes the “lemon-tarts,” the women who were young, the live-in girlfriends or subsequent wives. But he notes that one type of woman was missing: “[N]o one ever invited … Mother.”

This passage has stuck with me, particularly when I consider how our notions about both motherhood and fatherhood have been diluted.

Frequently, both women and men are valued for their career accomplishments rather than their relationships as spouses and parents. The outsourcing of conception, pregnancy and child rearing also make it more difficult to understand the irreplaceable contributions of a mother or father. When more than 40% of all births are to single mothers, fathers seem to be anything but essential. Divorce, all too commonplace, makes both mothers and fathers seem interchangeable and certainly not unique. All of this has paved the way for same-sex families in which two mothers or two fathers substitute for the natural complementarity of one mother and one father.

And yet we still have Mother’s Day and Father’s Day as two separate days to celebrate the indispensible contributions of each parent, a societal reminder that being a mother or a father isn’t simply about what that particular person does, but about who they are.

Focusing on Mother’s Day in particular, many factors over the past decades, ranging from materialism and the birth-control pill to various strains of feminism, lessened our sense of a mother as a person and made her just a functionary. Some mothers have become used to apologizing for “just” being stay-at-home moms. At social gatherings, a woman can be introduced as a mother only to receive the stunningly obtuse follow-up question, “Do you work?”

Women representing different strands of feminist thought, including those who distance themselves from any type of feminism, struggle with this tension. I had a unique experience of this several years ago, attending a conference on maternal feminism at Barnard College in New York. Participants were challenged to see if they could agree that, for many women, maternity is a defining part of their identity.

Unfortunately, even among a room full of women, there was not much consensus about the place of maternity in our lives. Yet one woman stood up and made this point: “Everyone in this room can probably make a better peanut-butter-and-jelly sandwich than I can, but not one of you has the relationship that I have with …” And then she went on to name each of her seven or so children.

Her comment focuses on the question of who a mother is rather than what a mother does.

There are many, many excellent mothers who do things very differently. Some work outside the home; ohers don’t. Some plan amazing parties for their children; others do the scaled-down version. Some involve their children in lots of activities; others just open the door and say, “Go out and play.” Some are strict disciplinarians; others not so much. The differences among mothers are countless. And yet all these differences don’t necessarily tell us whether or not a woman is a good mother. That’s defined by the relationship she has with her child, by who she is to that child.

First and foremost, a mother is someone who has nourished and nurtured the child since he first existed. By design, a woman’s body is meant to facilitate this close relationship that no father can experience, no matter how close he may be to his child. In the case of a woman who is not the biological mother of the child, I would argue that her body is still disposed to this specific type of relational intimacy, even if she herself is not able to conceive.

In no way do I want to suggest that men or fathers cannot be nurturing and loving. They certainly should be; but they do so differently from women. The sexually differentiated body, which physically and anatomically distinguishes women and men, accounts also for many of the behavioral differences of the sexes.

Aside from our bodies, women and men are the same — we have the same type of soul and are made equally in the image and likeness of God. But we experience the world through different types of bodies, impacting every aspect of our being, including our relationships, as seen most clearly in the relationship between mother and child (Catechism, 2331-2336).

There’s a tendency in discussions of this type to want to define everything very tightly. However, that also runs the risk of creating gender stereotypes, which continue to be challenged because they do not reflect wholly the human experience — whether it's the mother created by 1950s’ advertising or the idiot father in most family sitcoms of the past 20 years.

And yet there’s a tension when we don’t recognize the realities of our uniqueness. Despite the egalitarianism of our society, where women and men can do many of the same things, they cannot be parents in the same way, even though families increasingly exist incompletely, lacking both a mother and a father. They may both excel at changing diapers, drying tears and helping with homework. Or not.

Regardless, each parent has a fundamentally different relationship to the child, one which no one else can replace, which is essential to their identities and should be welcomed throughout society, even at elite parties.

Mother’s Day is an opportunity to celebrate one of these parental relationships. Father’s Day hosts the second celebration. Both are desperately needed.

Pia de Solenni is a moral theologian and cultural analyst. She writes from Seattle.