Julian Rotter was a very smart psychologist who studied how people think and behave. He was famous for coming up with a theory called "locus of control."
This theory is all about who we believe is in control of the things that happen to us. Some people think that they are in control of what happens to them (they have an internal locus of control), while other people think that outside forces are mostly responsible for what happens to them (they have an external locus of control).
For example, if you think that you do well on a test because you studied hard and were prepared, you have an internal locus of control. But if you think that you did well on a test because the questions were easy, you have an external locus of control.
Rotter believed that which of these two types of locus of control people have can affect many areas of their lives, like how they cope with stress, how they approach challenges, and how they set goals for themselves.