psychology has gone through a timeline of different phases and leading perspectives

Historically, psychology has gone through a timeline of different phases and leading perspectives, though there has been overlap and earlier theories often continue to be influential in later years. Of course, all of these major perspectives have some value and have contributed to our understanding of human behavior, but which do you think made the most important contribution along the way? Bring a classic or highly influential reference from your “favorite” perspective in psychology (attach copy or paste working link), and explain what it says and why you think its contribution to psychology was so important.
Note: In grading, I look for the quality of your answer, not the quantity of words, so your answer can be any length as long as you fully explain what you are talking about. 1 paragraph is fine (Links to an external site.)
Positive Psychology seems too new to be included in the history of the field, but it’s already been quite a number of years that it’s been a popular new perspective.
Watch the Module 1 Video featuring Martin Seligman telling about Positive Psychology. Positive Psychology is one of the most venerable “off-mainstream” perspectives in psychology, criticizing the field for being too negative because it’s too focused on dysfunction.
Do you think this is a valid concern? That is, do you think Psychology puts too much focus on dysfunction and its recovery? After all, that’s the major application where Psychology can be useful, so why wouldn’t it be a main focus?