What is psychology? To understand the basis of perception, memory, creativity, fear, love, anxiety, addictions, and much, much more, psychology is the scientific study of mind and behavior. But studying the intersection between the mind's inner experience with the outer expressions of behavior is never simple.

The brain is one of the most complex known pieces of matter. Many basic ideas we still examine in psychology can be traced back to the theories of two Greek philosophers from the fourth century BC, Aristotle and Plato.

Even though their psychologies aren't a psychology you'd want to run today in psychological science, what you get out of the Greeks, like Aristotle and Plato, was a very logical, analytical form of psychology that treats behavior as something that isn't particularly mystical at all. You can analyze reasons for action, types of causes of an event.

Plato was what we call a nativist. He believed that certain kinds of knowledge are innate, whereas Aristotle believed that all knowledge is acquired through experience, which we call philosophical empiricism. Although few modern psychologists would tell you that nativism or empiricism are entirely correct, the issue of just how much nature and nurture explain any given human behavior is still very much a matter of study and debate.

The story of psychology continues in 17th century France, where Rene Descartes, trying to understand how the inner world of subjective experience and the external physical world are linked, argued that body and mind are fundamentally different things. This theory, called dualism, was challenged by Descartes's contemporaries.

Thomas Hobbes argued that the mind and body aren't different things at all, and that the mind is what the brain does. Other thinkers expanded this idea. Franz Josef Gall thought that brains and mind were linked, but by size rather than any one gland.

And this developed in an idea called phrenology— which was serious at the time, and in some ways, isn't 100% wrong, but was pushed so far that it now looks absurd— imagined, in fact, with pictures of the skull with little pieces of human capacity one after another— insightfulness and intuitiveness and compassion— that each human capacity was associated with a part of the brain.

Though perhaps a detour in the study of the mind and behavior, interest in the brain is growing. Other French scientists were beginning to link the brain and mind in a more convincing manner. Pierre Flourens and Paul Broca were the first to demonstrate, through surgical techniques and post-mortem examinations, that the mind is grounded in a material substance, namely the brain.

Their work set the foundation for the scientific investigation of mental processes. By the mid century, psychology was emerging as a unique and distinct field. Major contributions from two German scientists trained in the field of physiology, Hermann Von Helmholtz and Wilhelm Wundt, led to the first ever psychology department at a university.

Structuralism was one of the early schools of psychology. One of the main players in it was a guy named Titchener. Titchener had been a student of Wilhelm Wundt. And the concern there was with the structure of mind. So it wasn't so much with what does the mind do, but what is the structure?

While structuralism had a deep influence on the development of psychology as a scientific discipline, skeptics eventually overtook the debate, chief among them a man named William James, who would go onto define a whole new school of thought. He disagreed with Wundt that consciousness could be broken down into its constituent parts, and instead argued that consciousness was more like a flowing stream, serving to adapt people to their environments. So instead of studying the mental structure, he studied mental function— the study of the purpose mental processes serve.

Functionalism refers to things people do, try to get done— functions and any functional explanation. You're imagining that there's some thing that could be carried out. And you're asking questions about how and how well it gets carried out.

Himself deeply influenced by Charles Darwin's theory of natural selection, William James applied the evolutionary perspective to psychology, arguing that mental abilities must have evolved because they were adaptive.

In the late 1800s, Sigmund Freud arrives on the scene in Europe. And Freud has his own ideas about mental illness, about the origins of mental illness.

Freud's influence ran deep, but his ideas have been significantly diminished over the past 40 years. For one, his view of human nature was a dark one. His ideas were hard to test, and eventually, a new school of thought emerged— the humanistic psychologists, with more of an emphasis on the positive potential of human beings.

Freud's ideas continue to bear significance for modern research. But testability has been the crucial necessity for all psychological researchers since Freud's era. Influence by the classic work of Ivan Pavlov, whose famous dog experiments established the concepts of classical conditioning, John B. Watson and B.F. Skinner were the champions of the school of thought dubbed behaviorism.

It was concerned with empirical studies of how animals learn and adapt based solely on observing their outer behavior. Behaviorism did allow psychologists to measure, predict, and control behavior to an extent. But it did this by ignoring some important things— the evolutionary history of the animal at hand, and the inner mental processes that thinkers such as James and Wundt had been so fascinated by.

And so, behaviorism faded to an approach that took these challenges head on. This was cognitive psychology. Rising to prominence through the 1950s and '60s, cognitive psychology is the study of mental processes, including perception, memory, and reasoning.

The shapers of this revolution in psychology were Wertheimer, Broadbent, Nicer, and Noam Chomsky.

Chomsky looked at the development in the life course of language, and observed that things are happening so quickly that they're inexplicable unless you imagine that language is kind of wired in by evolution.

Inspired by the attempts of Karl Lashley, a former student of John B. Watson, to surgically study a rat brain and locate a site of learning, the cognitive psychologists would spawn the cognitive neuroscientists, who, using new methods of brain imaging, would be to first to study the link between cognitive processes and brain activity.

Today, psychology covers a vast range of territory. Evolutionary psychologists, such as John Garcia, and biologists, like E.O. Wilson, played a large role in studying behavior from the perspective of millions of years of evolution that has shaped it. Social psychologists study interpersonal behavior— Solomon Asch, Kurt Lewin, Gordon Allport, important thinkers who studied the pressures and influences on the brain from the perspective of its place in a complex society.

Psychology is the marriage of our oldest questions— who are we, and what are we here for?— with our newest way of getting answers— science.

It's a field that has not gotten smaller. It's gotten bigger. And there have been more and more connections made with other fields, with different areas of biology, in particular.

It promises to reveal exciting truths about human behavior, about human beings, about the mind, and about the heart, about how we interact with each other, and who we are between our ears.

No matter what your approach, the desire to understand the mysteries of human behavior is the common pursuit of psychology.