The concept that sex is the main obstacle to platonic friendship is so familiar, it might seem timeless. It's endorsed by Dear Prudence readers and by Hollywood. It's why it was awkward for me to sleep on Jeff's trundle bed and took the M79 home, instead. At heart, the "sex problem" is a "man problem": Even if women can repress their sexuality, the story goes, men can't. That's the notion behind a hokey meme several readers sent me called the Ladder Theory, which posits that men have a single "ladder" for all the women they know, with those they actively want to bed at the top rung and those they would screw only if extremely drunk at the bottom. This meme, in turn, corresponds to well-publicized claims that the "sexual pursuit area" of the male brain is 2.5 times larger than that of the female brain and that testosterone drives men into a sort of chemical trance when they see a pair of breasts. How could a beast like the human male, whose mind turns to pornographic pulp whenever a female enters his frame of vision, maintain a friendship with a woman?

And yet, the apparently insurmountable "sex problem" was once a secondary issue. Before the advent of modern feminist thinking, before two world wars opened up employment, and before institutions of higher education diversified, arguments against the feasibility of platonic friendship emphasized female incapacities rather than male urges. The logic, essentially, wasn't that men and women chatting together would get distracted by the prospect of physical contact, but that they wouldn't have anything to chat about.

Aristotle, who wrote on friendship at length in the Nicomachean Ethics, mostly excludes women from the discussion. And while he grants that friendship is possible between husband and wife, he specifies that marriage is an unequal and therefore imperfect relationship—it's comparable to the bond between ruler and subject. The many Western scholars who turned to the topic over the next few thousand years tended to agree. In de Amicitia Cicero argues that friendship exists between "good men," with the implication that women can't rise to the occasion. "[T]o tell the truth," Michel de Montaigne wrote in the 16^th century, "the ordinary capacity of women is inadequate for that communion and fellowship which is the nurse of this sacred bond; nor does their soul seem firm enough to endure the strain of so tight and durable a knot."

Montaigne's philosophy reflects, or maybe rationalizes, a basic reality: For most of Western history, society was not built to foster non-romantic attachments between the genders. Women were relegated to domestic roles, without access to education, and most cross-sex mixing was reserved explicitly for courtship. There were great, celebrated friendships between men, in fiction and in real life: Achilles and Patroclus, Thomas Jefferson and John Adams, Karl Marx and Friedrich Engels. And between women: the biblical matriarchs Ruth and Naomi, the suffragettes Elizabeth Cady Stanton and Susan B. Anthony. But as a result of structural inequities, there were very, very few Jeffs-and-Juliets  prior to the 20^th century. (My take here is not original: I'm drawing on research by William Deresiewicz, the sociologist Ray Pahl, the historian Theodore Zeldin, and the writer Lisa Gee, among others.)

If you look hard enough for historical male-female friendships—especially in rarefied artsy circles—examples do spring up here and there, like weeds through cement. Yet the men and women who did manage to find each other, and take nonromantic pleasure in each other's company, encountered such social hostility that it was difficult for them to form lasting bonds.

Take the story of the Bolognese seamstress Angela Mellini, who walked into the church of Santo Spirito one morning in 1696 and described the strange religious visions she'd been having to the attending priest, Giovanni Battista Ruggieri. He must have been taken with her story, because he invited her to become his penitent. In late medieval and early-modern Europe, this was a fairly common arrangement: A conscientious Catholic might confess three or four times a week to a specially chosen spiritual director. Angela accepted.

The next stage in their relationship was more unusual: As historian Jodi Bilinkoff describes in her excellent book on the confessor-penitent relationship, Angela somehow intuited that Ruggieri was guilty of a "sin against chastity" (probably masturbation) and decided, through prayer, that it was her responsibility to help. Shocked by the accusation, Ruggieri called Angela a gypsy, but then conceded that she was right and turned to her for assistance. At that point, their routine changed. Ruggieri would listen to Angela's confession and grant his blessing, then they would switch roles. He would call Angela "mother," confess, and receive her blessing.

A master and his subject had become peers, depending on each other for guidance and assistance. But this friendlike reciprocal attachment proved too outré. In 1698, one of Ruggieri's other female penitents became suspicious of the relationship and denounced the couple to the Inquisition. The inquisitors perceived their role-swapping as a threat to male clerical authority and sent Ruggieri into exile. If Angela and her priest were ready for something resembling friendship, their community was not.

Social mores interfered in more subtle ways, too, in early-modern Europe—not preventing friendships outright, but affecting the way men and women treated one another. The great critic Samuel Johnson is said to have been "friends" with Hester Thrale, an 18^th-century diarist. Johnson first met Hester and her husband, Henry, in 1764 at a dinner, then started coming by nearly every Thursday. When the Thrales moved out to a country estate, Johnson was their constant overnight guest and did much of his writing at a desk specifically set aside for him. Under cover of a three-way relationship, it was Johnson and Hester who had the deeper tie—they translated the Roman philosopher Boethius together, in alternating verses. In her diary, Hester wrote of their "mutual regard … founded on the truest principles religion, virtue, and community of ideas."

In spite of this regard, Johnson never put aside contemporary notions of the proper place for women. Henry and Hester fought often, and Johnson always took Henry's side, on the principle that a wife should be subservient to her husband. A hypochondriac, Johnson was constantly asking for Hester's ministrations. Once, Johnson convinced himself that he was dying and she told him that while danger wasn't imminent, he might as well face the truth that he was no longer young. Johnson found this logic insensitive: "And this is the voice of female friendship I suppose, when the hand of the hangman would be softer." Johnson wasn't simply an asshole—like most men of his time he expected the women in his life to provide domestic comforts. He turned his partner-in-translation into a maid, and the lopsided nature of their relationship makes it look at best like a proto-friendship, an undeveloped precursor to the 20^th- and 21^st-century versions.

The literary critic C.S. Lewis wrote in his 1960 book, The Four Loves, that "Where men are educated and women are not, where one sex works and the other is idle, or where they do totally different work, they will usually have nothing to be Friends about. But we can easily see that it is this lack, rather than anything in their natures, which excludes Friendship; for where they can be companions they can also become Friends." With reverse logic, the suggestion is that men and women have a natural impulse to develop intimate, nonromantic relationships with members of the opposite sex. But until the last century or so, it was quashed by structural inequities—and by the philosophies that justified these inequities.

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