In our society, the stranger represents the unknown in the equation of our interactions with others. Is the stranger going to help or harm me? Because we do not know what to think, and because we live in a media environment that never misses an opportunity to warn us of the dangers of the street, of the night, of young men who wear hoodies, it's more prudent to assume the worst: The stranger is a robber, a rapist, a killer. Our defenses go up. But anyone who lives in the city is actually more dependent on strangers than on family or friends. Strangers grow our food, package our food, prepare our food, serve our food. They manage the quality of the water we drink, drive our children to school, deliver the mail, fly our planes. Without strangers doing all of these useful things for us, we would be nothing but savage, wild animals with nowhere to run or hide.

You might argue that these strangers are only helpful because they are paid to be. If they do not help me, they will not get paid. If they don't get paid, they will have no money and suffer the pains of poverty. Adam Smith, the father of this kind of market-oriented thinking, said this in The Wealth of Nations. If, for example, a shopkeeper is not friendly to his/her customers, many of whom are strangers, the shopkeeper will lose business and end up poor. But the usefulness of strangers is much deeper than the logic of the market. It is, I think, a central part of human sociality. Humans are made for strangers; we have the ability not only to trust them but also to endure them. As the great sociobiologist Sarah Blaffer Hrdy pointed out at the opening of her book Mothers and Others, a planeload of humans is not the same as a planeload of chimps. The former can easily (and usually do) fly for 16 hours without an incident; the latter, however, could not "disembark [a plane] with all 10 fingers and toes still attached, with the babies still breathing and unmaimed. Bloody earlobes and other appendages would litter the aisles. Compressing so many highly impulsive strangers into a tight space would be a recipe for mayhem."

The first time I used SideCar, a new app-enabled car-sharing service that began in San Francisco and started operating in Seattle four months ago, I was excited by the fact that the driver was not only a complete stranger but he also owned the car. I was entering the stranger's private space. This is not what happens when I enter a bus; the driver doesn't own it, and so the bus is a neutral space. With SideCar, private space becomes connected with public space. Indeed, the blurring of these usually distinct categories produces new and unexpected feelings. It's kind of like walking into a stranger's house, sitting on their couch, and watching TV with them. Of course you feel a little odd (is this intrusive? How do we talk? What is the structure of our relationship?), and it takes some effort to find the appropriate attitude. My driver that first night was a white man named Lee (the app gave me his name, a picture of him, and a picture of the kind of car he was driving—a Mitsubishi i electric vehicle); he is a systems administrator at Google. He also has another car that he rents out through RelayRides. He is a member of the Seattle Transit Riders Union. He is a drone pilot. He spoke to me not like a person who is working but like a person I met at a party. But at a party, we always ask the stranger we meet in the kitchen how they know the host. What connects him to us? But all that connected me and Lee was my need to get home and his ability to meet this need.

To confuse matters even more, I sat in the front seat. The general understanding with SideCar is that it's not cool to sit in the back—that's what cabs are for, and SideCar is different from cabs. "Sometimes, I will pick up two people who want to talk about things, and so it's cool if they sit in the backseat," explained Lee. "But if you are alone, I prefer you sit in the front."

After our conversation drifted this way and that, it finally settled on politics. We talked about the need to share things. We live in a society that has too much stuff, and this way of living is simply unsustainable. Lee is a citizen of the green city. A big part of his involvement with SideCar is it reinforces an ethic of urban environmentalism. When we reached my destination, the app on the phone suggested I give him $14 as a donation (the ride was from Capitol Hill to Columbia City). It was up to me to pay more or less or none of this amount. There was no haggling, no fuss about cards or cash. I just quietly pressed the red button, the app did its thing, Lee was paid $14, and I climbed out of the car without saying a word about money.

Kelli picked me up on Sunday afternoon from the Twilight Exit. She wore a hat and sunglasses, and she arrived in a car that was not pictured on the app. This produced some confusion, but she explained that her usual car was in the garage. I sat next to her. Between us was a gadget that I did not recognize. It was being recharged. Did it have something to do with SideCar? No, she told me, it was a recorder that she uses for band practices. She is in a band? Yes, she is. What is the band? An all-women big band called the Mood Swings that play swing-era jazz. (From the band's website: "2 cups Sammy Nestico, 1 1/2 cups Duke Ellington, 1 cup Gershwin, 1 cup Latin Spice, 2 tablespoons Funk, a pinch of attitude—blended all together.") Kelli is the drummer of this band. Kelli also offers karaoke for her passengers. "It's on my iPad. I put it on the dashboard, and you can pick a song and sing as I drive." Some drivers also have bottled water, or candy, or condoms, I'm told. It's all very social, very useful, very practical. I gave Kelli $11.

Willie picked me up from the Quarter Lounge on a Friday night in a 2012 Nissan Sentra. He works in social services. He is black American. He has plans to study theology. He has been with SideCar for three months. What he doesn't get is why it took people so long to come up with this kind of service—connecting people with cars to people who need rides. His guess? We live in a society that sometimes makes the most obvious things so hard to see.

This was the substance of the conversation I had with Willie, who is very friendly and thoughtful: What made SideCar possible are smartphones. Without this technology, it would not have been possible to keep people in check and ensure the safety of both drivers and passengers. The SideCar app keeps track of you and the person who is driving you. And at the end of the ride, you rate the driver and the driver rates you, and if you give your driver a rating of less than four out of five, the app will ask if you'd like to block this driver from seeing future ride requests from you. All of this is nothing but the appropriation of corporate quality-control techniques and police surveillance technologies to make a new form of communal sharing possible. This, I think, is the genius of the service: It uses systems of control to liberate us from a society that systematically encourages self-centered, individualistic, antisocial behavior (look at the success of our bankers), a society that for years has generated nightmare after nightmare around the figure of the hitchhiker (usually a murderer who has just escaped from prison), a society that's made us fear the most useful person known to humans: the stranger. I paid Willie $13 and, as with the other strangers who drove me around town, gave him the best rating possible: five stars.