The correct answer is introspection.
Introspection is the examination of one's own thoughts and feelings. It means observing one own's mental state which many believe means examining the souls as well.
It is the process of looking inward and reflect on our emotions and memories.
This research technique was first developed by the psychologist Wilhelm Wund. His technique involved training people to carefully and objectively as possible analyze the content of their own thoughts.
Edward Titchener was a student of Wilhelm Wund. He was interested in the individual components that comprises conscious experience and he developed the psychological theory of structuralism.