In the late nineteenth century the popularity of psychology needed the respectability of a more scientific base to its teachings. John Watson was a psychologist interested in child development and he brought behaviorism to the fore in his (now suspect) studies.
For Watson, introspection, the experimental self-observation of Wundt's technique was highly unscientific. Wundt wanted to train people to analyze the content of their own thoughts. Watson stated this was impossible in a scientific manner: it could not observed, it could not reliably be recorded.
John Watson, the popular colleague of Sherlock Holmes had numerous theories. Many of them were grounded by valid reasoning. For him, Introspection was unscientific. Introspection means knowing your own self. This means that behaviorists like skinner reject it and would not consider it as something you can observe.
Similarly, john Watson also considered it to be an impossibility. There is a limit to which you can observe your own mental and emotional processes. Some might even consider it unhealthy. Knowing your own self was unscientific s you cannot truly know yourself. You can only know degrees of yourself and still be surprised by the daily ventures of your thoughts and emotions.