How much does one has to "know about" a person to "know" a person? When does a stranger become an associate or acquaintance, an associate or acquaintance become a friend, and a friend become an intimate? When is a stranger no longer a stranger? How does one know when one is "close" to someone? Those questions have bothered me for quite some time. If I read a biography of a celebrity whom I have never met, and I am able to memorize the entire contents of the biography, could it be argued that I "know" the celebrity without actually having never met the celebrity? Since no human being has complete knowledge about any other human being, do we truly know anyone except for ourselves?

I think the best answer is that there's no one answer.

Let's start with the easiest of your questions: you've read a biography of someone you've never met. Do you know them? Most people would say "No" because when we say things like "I know Robin," we generally mean that we are acquainted with Robin--have actually met Robin. Knowing about someone is knowledge by description but not by acquaintance, to borrow Bertrand Russell's terms.

In the other cases, there's no simple answer because the terms "mere acquaintance," "friend," "close friend" and so on aren't precise; there's no cut-off. It's like the case of baldness. There's no exact point at which a formerly hirsute person becomes unequivocally bald.* The case you've focused on is an instance of a very general phenomenon. Some people are definitely tall, some are not tall, and some are on the border. Some bananas are definitely ripe, some are not, and for some there's no definite right answer. As you can see, it would be easy to make a very long list of similar cases.

I'd suggest that rather than looking for sharp borders, it's more interesting to ask what typically goes with the difference between being a friend and being a mere acquaintance, and so on. For example: if two people take no pleasure in one another's company, it would be odd to call them friends. If John knows that he can turn to Mary when he's in trouble, and Mary knows the same of John, it would be strange to describe them as mere acquaintances. You might be able to think of exceptions. That's fine. It's what we might expect if there really are no sharp boundaries.

As for whether we truly know anyone except ourselves, it depends on how much stress we put on the word "truly." But I'd add a caution: there's plenty of room to doubt that we know ourselves as well as we think we do. Self-misunderstanding is a pretty common phenomenon, and sometimes an astute observer's view of us be more accurate than our own.

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*Some philosophers dispute this, but that's another long and complicated story. I think I'm on safe ground when I say that most philosophers think some terms really are at least somewhat vague.

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