Let's say you're a huge fan of a celebrity, like Johnny Depp. You know all the facts about his life: when he was born, where he went to school, his favorite hair gel, and so on. You've seen all his movies, you rank 21 Jump Street as the greatest TV show ever, and you named your firstborn Sparrow.
You might know everything about him, but would you say you know him? Probably not, though I assume you'd like to. And what would it take to know him, rather than just know about him or know of him?
In a recent paper titled "Knowing Persons" in the journal Dialogue, philosopher David Matheson of Carleton University asks that very same question. He calls the type of knowledge one might have about a celebrity impersonal knowledge, while the type that would lead you to say you "know somebody" he calls personal knowledge. But what is it about them that makes knowing about someone so different from knowing them?
Matheson considers and dismisses several ideas suggested by philosophers in the past, such as that personal knowledge comes from being acquainted with someone, or being able to recognize someone, or having the capacity to interact with someone smoothly. He argues that none of these are sufficient to explain the depth of "knowing someone," as well as curious things about this type of knowledge, such as its "limited transferability." For instance, if your friend knows some impersonal knowledge about Johnny Depp, like his birthday, she can share that with you and you'll know it as well, just like any other fact. But if your friend knows Johnny Depp, she can't share that with you—she can tell you everything she knows about him, but you still won't know him yourself.
Why not? It's because of what Matheson calls the "communication account" of knowing a person, by which you only know someone if he or she actively shares information with you, particularly intimate, private information. For instance, you may be able to find out online what Johnny Depp's favorite movie is, but he were to tell you himself, perhaps with personal insight into why it's his favorite, or where he first saw it, that would give you reason to say you know him (at least a little). So there is an active element on the part of the person you come to know, and it is this aspect of knowing someone, the communication between you, that makes knowing someone so meaningful, and by extension, hard to share with a third person who isn't in on the communication.
This also explains why it feels special to know someone, rather than just know about them: the fact that he or she has opened up to you personally. If Johnny Depp tells you about his favorite movie, rather than talking about it in a magazine interview that's read by millions of people, that means he's judged you worthy of sharing that information, implying that you can appreciate it. And in cases of more sensitive information, personal communication also carries a great deal of trust and vulnerability: just think of how much it means when someone tells you, "you're the only person I've ever told this too."
So the next time someone complains that "you don't know me," just reply, "so tell me something about yourself." Even if it's Johnny Depp—it's OK, really, I know him...