I like my close friends a lot, and yet, on an almost daily basis, they shocked me. I have a friend who thinks voting is a waste of time; I have another friend who never takes any arrangement to meet at a given time and place seriously.

It’s generally held that friends are people with whom we choose to develop relationships because we find their personalities agreeable, or similar to our own, but experience regularly is opposite to this. What is a friend, really? All that one can safely say is that a friend is someone one likes and wishes to see again.

The truth is that we don’t know our friends. Many studies show that we tend to think our friends agree with us more than they really do. The striking part is that the problem doesn’t appear to lessen as a friendship deepens. When the researchers Michael Gill and Bill Swann questioned students sharing rooms, they found that, as time passed, people became even more confident in the accuracy of their judgments about the other, and yet, in reality, the judgments grew no more accurate. Two people might become dear friends, yet possibly know little about many areas of each other’s inner lives.

This seems strange, until you consider, that many of the benefits that friendship provides don’t necessarily depend on perfect familiarity; they come from something closer to trust. Friendship may be less about being drawn to someone’s personality than about finding someone willing to keep you company, or lend an ear. A friend provides the “social-identity support” we desire. You needn’t be a close match with someone, nor deeply familiar with their mind. We don’t base friendships on what we learn about people; we decide what to learn about people, and what to ignore, based on having decided to be friends.

Perhaps there’s something moving about viewing friendship as an agreement to keep each other company, ignore each other’s faults and not probe (刨根问底) too deeply in ways that might weaken the friendship. Perhaps a true friend is someone who doesn’t ask many awkward questions.