There are two basic aspects of identity and a third in reality: what you do, who you think you are, and how other people see you. We sometimes deny the third, but there’s no end of research that shows how people perceive you affects how you perceive yourself and how you act.
For the blog post, let’s confine ourselves to the first and second.
The former is the old ‘someone who’s nice to you but mean to the waiter isn’t a nice person.’ Let’s throw in that person might think of themselves as a nice person. However being mean is a habit, and habits become default behavior. If you want to know if someone’s going to be nice to you in the future, how they habitually act to others now is probably the best way of predicting that. There’s two-way dependency wherein who you are is and is determined by what you do.
If you want to understand someone, you must know how they see themselves. That’s the filter through which they will perceive all of your interactions. If the interactions themselves are of import, if the relationship matters to you, you need to know what’s happening on their side. Imagine a bridge over a canyon. You can’t build just one side of a bridge. Without a foundation for the bridge to land, the bridge will collapse, and with people, that foundation is often who they think they are.
To understand someone, you need all three.