Psychology has existed in its popular form as long as human language has existed. Being able to rely upon the cooperation of your fellow caveman would have been even more vital than it is in the present day- human beings therefore developed a curiosity about the motives and nature of other people very early on. In the days before TV, humanity would have needed to turn tribal life into one continual soap opera simply in order to survive the boredom. In its purest form soap is a lesson on human nature- it is possible to look at the characters in the soap and compare there own actions to those you would have chosen yourself. In this way we can develop an idea of the mental processes that drive them.
In order for Psychology to be a proper subject of study it must be a science- there is no point is studying somebodies woolly generalizations if they are no more soundly based than ones own.
Psychologists must believe in the scientific nature of psychology or the entire subject ceases to exist! If everything is an opinion then there is no real subject of Psychology and therefore no reason to consult a Psychologist.
The problem with calling Psychology a science is this: human beings are emotionally involved with the study of themselves. Every political philosophy has created its own psychology because of the inability to separate the two.
Some psychologists have carried out IQ tests and found clear differences between different races. This creates great upset and yet the results are never checked in the same way as experiments in physics are.
If you or I were to discover a new particle in physics the research community would accept this quite calmly- they would reserve judgment until the results had been duplicated in another lab.
In psychology we see a more 'human' response. Researchers who come across inconvenient results are called frauds and their research is suppressed- but never checked for its truth. As a result we will probably never know for sure if such differences exist.
Psychology as a true science is still awaiting birth- but as a generally useful bunch of observations it already exists.