Male/female

"According to Aristophanes in Plato's Symposium, in the ancient world of myth there were three types of people," Oshima says. "Have you heard about this?"
"No."
"In ancient times people weren't just male or female, but one of three types: male/male, male/female, or female/female. In other words, each person was made out of the components of two people. Everyone was happy with this arrangement and never really gave it much thought. But then God took a knife and cut everybody in half, right down the middle. So after that the world was divided just into male and female. The upshot being that people spend their time running around trying to locate their missing other half."
"Why did God do that?"
"Divide people into two? You got me. God works in mysterious ways. There's that whole wrath-of-God thing, all that excessive idealism and so on. My guess is it was punishment for something. Like in the Bible. Adam and Eve and the fall and so forth."
"Original sin." I say.
"That's right, original sin." Oshima holds his pencil between his middle and index fingers, twirling it ever so slightly as if testing the balance. "Anyway, my point is that it's really hard for people to live their lives alone."

Kafka on The Shore, one of Haruki Murakami's book.