The third relational practice (yama, summary here) listed in the Yoga Sutras is asteya, or non-stealing. It's most obvious meaning is uncontroversial - don't take what isn't yours! If humans want to live together harmoniously, this rule is pretty essential. Most of us are past our shoplifting days, but things get slippery when we start to consider less concrete forms of theft. Have you ever claimed credit for an idea that isn't yours? Have you ever taken up a whole conversation just talking about yourself? Do you steal happiness from yourself by focusing only on what goes wrong? Do you use more water, food and fuel than necessary? Asteya calls us to become more aware of how we take or give to others.
But there is more to consider here.
If I am going to try to avoid stealing things, I have to discern what is mine and what isn't. To answer that question, I have to figure out who "I" am. According to yoga psychology, my sense of "I" is simply a motif of thought and feeling that arises in the stream of experience. It is not something solid and enduring, so how could it be said to "own" anything? Similarly, my body and all my possessions are standing waves of matter and energy, transient collaborations of elements and information. Can one wave on the ocean be said to own another?
From this perspective, to think of anything as "mine" in a permanent sense is a subtle form of stealing. To "take it personally" - success or failure, pain or pleasure, possessions or needs - is to take as mine something that is not. Asteya thus becomes another form of satya, the pursuit of an honest relationship to reality.