Psychology1 is the study of both internal cognitive and affective (emotional) processes, as well as the demonstrated behavior associated with those processes. There is a division between psychology applied to clinical treatment and psychological research, although the former generally involves research-based and evidence-based approaches. Psychotherapy focuses on the direct application of theory in a clinical setting, with the intention of bringing about behavioral and/or emotional change. Experimental psychology is focused upon applying scientific means to ascertain the foundational constructs that underpin both thought and action.
Most Universities today have two fields of Psychology curriculums for undergraduates. One is in the Arts, and the other in the Sciences. The latter dealing with the medical biological functions of the brain and central nervous system. The former dealing with the classical paradigms established through observations and theoretical reports of the past.
Although psychology is often mistakenly viewed as a young discipline, it actually finds its roots in ancient Greece. Aristotle is typically considered the father of psychology, and his treatise De Anima, Parva Naturalia, written circa 350 CE,  is widely regarded as the first codification of psychology as a formal discipline. 
A careful reading of Buddhist works also shows a clear address of psychological topics.  This is especially true of the Abhidhamma, a commentary on the sutras, where we come across explicitly psychological works. 
We must, in addition, consider the influence that the travels of Alexander the Great in the 4th century BCE and Plotinus in 242 CE to Northern India had upon Christian Mystics like Meister Eckhart and St. John of the Cross. These writers' subsequent influence on the Theosophical movement of the 19th century make the role of ancient thought in the development of the modern psychological paradigm evident. 
Psychology as modern science
Although a record of psychological experimentation dates back to Alhazen's Book of Optics in 1021,  psychology as a modern field of study can be traced to the year 1879, when Wilhelm Wundt established the first laboratory dedicated to psychological research at Leipzig University in Germany.  The American philosopher William James published his seminal book, Principles of Psychology , in 1890, laying the foundation for many of the questions that psychologists would focus upon for years to come. Other important early contributors to the field include Hermann Ebbinghaus (1850–1909), a pioneer in the experimental study of memory at the University of Berlin; and the Russian physiologist Ivan Pavlov (1849-1936), who investigated the learning process, and established the tenets of classical conditioning.
Modern and Post-modern thought
Wundt attempted to quantify human thought into basic elements; he was inspired by the success of chemistry and physics in describing the world by component pieces such as atoms and elements. In 1879, Wundt founded the first modern laboratory for the study of psychology at Leipzig University in Germany. Wundt's ideas were picked up by his student Edward B. Titchener, who coined the term structuralism to describe it. Structuralism would come to dominate the study of psychology for most of the late 19th and early twentieth century.
Structuralism's goal was to describe all of Wundt's elements of thought. It used a technique called introspection, where volunteers were trained to break down their thoughts into basic elements and were then shown various objects or exposed to various concepts and asked to describe the basic elements of their thoughts. For example, if shown a book a subject would attempt to describe the elements of what he is seeing by shape, and color but without reference to higher order concepts such as "binding", "cover" or "pages."
The use of introspection and structuralism's seemingly inherent subjectiveness created a backlash that developed into the concept of functionalism. This idea was most famously put forth by William James. It attempted to understand mind and behavior not as a "how" but as a "why." Functionalism was heavily influenced by Charles Darwin and the theory of evolution. It would later serve as the foundation for the behaviorist movement.
At the same time that structuralism and functionalism were being developed and debated, Sigmund Freud was developing his ideas. Psychodynamics had an enormous influence on psychology, and in many ways that influence continues to this day. While many of his ideas have been discredited, Freud was also the first to introduce several core ideas into the field. These included the concept of an unconscious mind being able to change and direct behavior, and that people go through developmental stages in which their mind and behavior change as they grow up and grow old.
At the turn of the century, Ivan Pavlov started his work on the gastric reflex in dogs. He discovered that a salivary response could be activated by a cue such as a bell ringing if that cue was linked to the presentation of food. This concept was developed into the idea of "classical conditioning." This work was absolutely pivotal in the history of psychology. It combined with the concepts developed in functionalism, and the field of behavioral psychology was born. John B. Watson is generally regarded as the founder of behaviorism. Behaviorism was, in many ways, a reaction to the introspective, subjective-based methods of structuralists and their goal of understanding the elements of the mind. Behaviorists abandoned the concept of mind as a scientific concept altogether. Instead they attempted to describe all behavior as merely the product of inputs from the environment and outputs from the organism. The mind was viewed as a 'black box' and never examined.
Pavlov, and later B.F. Skinner, as well as many others, were able to formulate precise mathematical descriptions of training regimes and reactions. It was hoped that psychology could became a hard science like physics, with mathematically rigorous theories. Skinner is most known for his work with operant conditioning or instrumental learning, where small changes in behavior are rewarded in order to create complex behavior. This is perhaps epitomized by the Skinner box, in which he trained pigeons to perform many complex actions for food rewards.
Skinner also thought that all of human behavior was a function of conditioning and instrumental learning. He wrote several non-fiction books such as Beyond Freedom and Dignity and several fiction books, including Walden 2, advocating the creation of Utopian societies based on instrumental learning.
During the 1960s, several developments altered the course of psychological research. The first was the creation of the computer and the ideas of Alan Turing. People quickly saw the analogy between computers and the human brain and thought that perhaps a mathematically rigorous concept of the mind could be developed. On another front, several experiments had begun turning up anomalous results. One of the most famous is the 'Garcia effect', where an animal that is exposed to a novel food and then made sick instantly learns to no longer desire that food; however, if it is a food the animal has had plenty of pre-exposure to, it does not instantly acquire the conditioned dislike. This instantaneous acquisition in one condition and not in another could not be explained without cracking open the black box of the mind and allowing it to affect behavior.
Noam Chomsky also challenged the behaviorists. Skinner's functional analysis of language described the acquisition of language as instrumental learning, but Chomsky provided counter-examples by which he claimed to show that Skinner's explanation was inadequate. Among the examples were errors that children make when acquiring their native language. Chomsky pointed out that children will often attempt to apply a grammatical rule universally and say something like "I runned to the store" - something that they could never have heard prior to producing it. An environmental explanation alone would not predict such errors. Perhaps more persuasive was the observation that children learn language at a rate that a functional theory could not account for: Although there is little to directly compare children with, they produce novel utterances at a rate that is obviously higher than what they happen to be exposed to. Attempts to model language acquisition on computers that could make similar errors required certain innate rule sets existing at the beginning of language acquisition. Many behaviorists objected that Chomsky had missed Skinner's point, but the 'generative' theory that Chomsky offered as an alternative to Skinner's functional one was widely influential, spawning the science of modern linguistics and thousands of research papers up to the present day.
Thus cognitive psychology was born as a reaction against behaviorism. Cognitive psychologists attempted to understand the 'black box' of the mind through computational analysis, modeling and rigorous experimentation. Also during this time, clinical psychology was having its own revolution with psychoanalysis being questioned by existential and humanist approaches.
Developments to the present
In addition to cognitive psychology, social psychology was also coming into its own. Stanley Milgram published his works on the Milgram experiment, which demonstrated that normal individuals would obey an authority figure and endanger the health and life of other people. This was also linked to Philip Zimbardo's work on the Stanford prison experiment, demonstrating the ability of college students to quickly turn into torturers. Other important concepts such as diffusion of responsibility and cognitive dissonance were being developed in experimental laboratories. In the 1970s, E.O. Wilson published his book on sociobiology; this work as well as much in social psychology was picked up by researchers with a strong biological and evolutionary influence. It ultimately emerged as the field of evolutionary psychology in the late 1980s and is currently one of the most hotly contested and productive areas of psychological research.
Technological developments during the last thirty years have also drastically altered the field of psychology. Computer processing power and memory increases have allowed researchers to develop models of many areas of cognition. More importantly, imaging techniques that allowed psychologists to non-invasively peer into the brains of living people and see what areas were functionally active have revolutionized much of psychology.
Clinical work on psychotropic drugs for treatment of various psychosis such as schizophrenia, bi-polar disorder and depression have also allowed researchers to start to understand the neurochemistry involved.
Paradigms of Psychology
Psychology has traditionally emphasized the concept of different major paradigms for the different theories on the underlying uniting concepts for the field. These paradigms often represent both historical movements as well as themes in which current practitioners and researchers work in. Some of the major paradigms in psychology have included:
- Cognitive psychology- this school of thought has a strong focus on understanding the mechanisms of the mind in order to explain behavior. It often uses analogies to computers to accomplish this task, one of its great themes is the idea of the Computational Theory of Mind.
- Behavioral psychology - this school of thought views the mind of the organism as a 'black box' which is impossible to describe empirically, therefore the focus is on understanding behavior as a by-product of environment. Ivan Pavlov, JohnB. Watson, and B.F. Skinner epitomized this philosophy with their focus on classical conditioning and instrumental learning.
- Psychoanalytic psychology - this school of thought attempts to understand mind and behavior as a product of the unconscious. It is most closely linked with the ideas of Sigmund Freud and Carl Jung. Freud believed that much of behavior is related to repressed sexuality, while Jung and others extended his ideas to include other forms of repression. Psychoanalytic psychology is primarily used in clinical settings though it is not unheard of for researchers to appeal to Freudian concepts.
- Existential psychology and Humanistic psychology - while these schools of thought differ in some fundamental ways, they are often linked together because of their focus on differences and the importance of the individual over generalized rules. Existential Psychology emerges from French Existential philosophy, most notably linked to Jean-Paul Sartre. It places great importance on existential angst as an inevitability of existence, and that the role of psychology is to help individuals recognize their angst and come to terms with it. Humanistic approaches to psychology are closely linked to the ideas of Abraham Maslow and his idea of the Hierarchy of Needs. To Maslow, psychological illness is a consequence of the difference between the idealized self and the actual self, the role of the psychologist is to help the person either adjust his idealized self image or improve his actual self. Carl Roger's ideas of Client Centered Therapy are also closely linked to humanistic approaches.
- Evolutionary psychology and Biologically-based psychology - these approaches attempt to understand mind and behavior as products of biological interactions and evolutionary history. This school of thought is heavily researched-based. Evolutionary psychology is a relatively recent development, and many of its proponents believe that a strongly biological approach to psychology may ultimately serve as a unifying principle for the field of psychology.
Fields of Psychology
- Clinical psychology as an applied discipline is oriented toward therapeutic intervention for individuals (often including work with groups) with psychological difficulties or disorders.
- Counseling psychology
- Educational psychology
- Forensic psychology
- Health psychology focuses on the impact of psychological factors on behaviors that are relevant to physical health. Researchers in this field study topics such as substance abuse, obesity, and exercise.
- Industrial/Organizational psychology
- Engineering/Applied Experimental Psychology is concerned with the interactions between humans and technology. This field is sometimes referred to as Human Factors. The field began during World War II when faulty designs in aircraft displays and controls were leading to crashes. The knowledge and methodology from experimental psychology was employed to correct and optimize these designs. The field quickly spread to other areas of the military. After the war, the field expanded to civilian applications. Given the ubiquity of technology in modern life, the field has become ubiquitous. Unfortunately, the field is not widely known to the public and people often blame themselves for difficulties they experience with technology rather than to faulty design and the failure to employ methodologies from human factors.
- Sports psychology
- Abnormal psychology - the study of abnormal cognition and behavior.
- Clinical psychology as a research field is the study of psychopathology: its origins, nature, and treatment.
- Cognitive psychology
- Developmental psychology - the study of how mental processes and behavior change with age. One version - "child" psychology - focuses on changes from birth through to adolescence; another version - lifespan human development - includes changes throughout adulthood and old age.
- Educational psychology - focuses on topics related to teaching, learning, and classroom-based experiences in general. Its most central areas are motivation, classroom learning processes, assessment of learning, social processes in classrooms, and diversity among learners.*Experimental Psychology
- Evolutionary psychology
- Personality psychology
- Social psychology - the study of social behavior or interpersonal interactions. Social psychologists study issues such as the way in which attitudes towards other people are formed, or the effect that our perceptions of another person's behavior have on our interactions with that person.
- Transpersonal psychology
- Psychometrics - the science of psychological measurement.
1From the Greek, psyche; literally, "to talk about the soul".
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- Omar Khaleefa (Summer 1999). "Who Is the Founder of Psychophysics and Experimental Psychology?", American Journal of Islamic Social Sciences 16 (2).
- Bradley Steffens (2006). Ibn al-Haytham: First Scientist, Chapter 5. Morgan Reynolds Publishing. ISBN 1599350246.
- Wilhelm Maximilian Wundt
- The Principles of Psychology (1890), with introduction by George A. Miller, Harvard University Press, 1983 paperback, ISBN 0-674-70625-0 (combined edition, 1328 pages)
- James, William (1890, 2007). The Principles of Psychology, Vol. I. Cosimo Classics:
- James, William (1890, 2007). The Principles of Psychology, Vol. II. Cosimo Classics: