Psychology is a fairly new science and its history is fairly short. The history of psychology is classified into 5 different eras.
The first era began with stone age humans using trephination (the carving of holes into skulls) to "remove evil spirits". These attempts show that the early humans were at least experimenting with ways of curing headaches, making it the first known instance of humans attempting to develop the modern field of psychology. Later on, Greek philosophers like Plato and Democritus thought about the relationship between thought and behavior. However, the first truly scientific studies of psychology began in 1879 with a man named William Wundt. He set up the very first psychological laboratory in Leipzig to study introspection. To do this he had subjects record their cognitive reactions to simple stimuli and began to develop a theory of structuralism, which is the idea that the mind combines subjective emotions and objective sensations. The first psychological textbook was developed by William James in 1890. James developed a theory referred to as functionalism, which describes how the structures identified by Wundt function in our lives. In modern times, the idea of introspection is disregarded in favor of theories that are better at explaining the human psyche.
The second era began with Gestalt psychologists like Max Wertheimer choosing to analyze the total experience of person rather than the individual experiences themselves, the total opposite of introspective theories. Gestalt theorists demonstrated that the whole experience of a person's life is much more than just the sum of its parts. Modern therapists use this Gestalt thinking by examining the context in which a client's difficulty occurs as well as the difficulty itself. Other than its contributions to therapy, gestalt psychology has had little influence on modern psychology.
The third era was entirely based on the ideas of Sigmund Freud. He revolutionized the field of psychology with his idea of psychoanalytic theory. Freud believed that people possessed an unconscious mind, which is a part of our mind over which we have no control and determines somewhat how we think and behave. Freud believed that this hidden part of our minds builds up via repression, or our pushing of thoughts that cause us great anxiety into our unconscious. Freud believed that understanding behavior required us to understand the repressed thoughts and emotions in the subconscious mind. However, some psychologists have criticized his ideas as being unscientific.
The fourth era was the era of behaviorism, the idea that behavior is only our response to stimuli in our environment and looking at the conscious elements is unnecessary. The idea of behaviorism was developed largely by John Watson, who studied Pavlov's drooling dog experiments and concluded that for psychology to be a science, it must limit itself to studying observable phenomena, rather than things like the unconscious mind. Another large figure of this area was B. F. Skinner who developed ideas of reinforcement in which environmental stimuli either encourage or discourage certain responses. Behaviorism still holds a large impact in psychology today.
The fifth or modern era is the era of mixed perspectives. Most psychologists do not attribute causes for behavior and human thought to any one theory, but many theories. Until more psychological evidence and theories are developed, it is unlikely that this will change.