Psychology is the scientific study of thought and behavior. This means asking why we do the things that we do and think the thoughts that we think. If you've ever wondered why you need to spend so much time sleeping, why visual illusions occur, or how to figure out whether or not that cute classmate has a crush on you, you've considered psychological questions. While current research may not be able to definitively answer the many questions we pose, it can certainly help us to investigate the plausibility of the explanations we generate.

You see, whether you study psychology formally or not, there are questions that you will undoubtedly confront throughout your life. We all wonder about the world we live in and in doing so, we ask the type of questions that psychology attempts to answer. We wonder about the causes of events and behaviors, why some people seem to dance on silver-lined clouds while others are left lamenting the rain, or how some otherwise-rational person can so fervently disagree with our own point of view. I mean, what is wrong with that guy?!

You shouldn't think of psychology as a separate subject, neatly compartmentalized from all others. What I hope this book will show you is that psychology is life. The study of psychology will lead you from statistics to sociology, philosophy to pharmacology, epistemology to economics, mathematics to meditation, and biology to basket-weaving. Attempting to understand our own thoughts and behaviors inherently ties us to all of human endeavor. Never can one say that the field of psychology isn't relevant to one's personal life. No matter what you're interested in, psychology can help you to understand it. And if you aren't interested in anything at all, psychology will have something to say about that too.

While psychology as a field of academic study may be relatively new, our natural human desire to understand the world means that we have been asking psychological questions since the dawn of our existence.

One of the key philosophical influences underlying research in psychology is the attempt to understand how we know what it is we know. Is our knowledge and understanding of the world inborn, or is it acquired through our experiences?

This question goes back thousands of years, long before scientific concepts of genetics or elite preschools. PlatoPlatoAncient Greek philosopher (427–347 BCE) who believed knowledge is innate — we are born with ideas already inside us, waiting to be rediscovered. (427-347 BCE) believed that we are born with all of our knowledge already inside us, simply waiting to be rediscovered. This concept that knowledge is innate or inborn is known as nativismnativismThe view that certain knowledge or abilities are innate — present at birth rather than learned through experience.. While there aren't many psychologists today who adhere to strict nativism, we still see this line of thinking in attempts to understand certain types of behavior. Are some people simply born to be sociopathic criminals or loving parents? Predestined for a life of worry-free bliss or chronic depression?

Or perhaps there's something about our experiences and living environment that shapes who we are? AristotleAristotleAncient Greek philosopher (384–322 BCE) who argued all knowledge comes from sensory experience — the foundation of philosophical empiricism. (384-322 BCE) challenged Plato with the concept of philosophical empiricismphilosophical empiricismThe belief that all knowledge is acquired through experience; we are born a blank slate (tabula rasa) written upon by the environment. — the belief that all knowledge is acquired through experience. We are born with nothing; a "blank slate" (tabula rasa) waiting to be written upon by our environment.

So while the question of nativism versus empiricism goes back thousands of years, we still don't have a definitive answer, except perhaps to say that most traits and behaviors arise from a complex interaction of both nature and nurturenature / nurtureThe ongoing debate over whether behavior is primarily shaped by genetics (nature) or environment and experience (nurture). Most psychologists today view it as an interaction of both. — genes and environment. It's not so much a question of "versus" as a question of how the two combine and collaborate to create each unique person.

Without a doubt, you're familiar with some of the work of René DescartesRené DescartesFrench philosopher (1596–1650) famous for "I think therefore I am" who proposed dualism — that the non-material mind and the material body are separate substances. (1596-1650). Perhaps you recognize his famous quote "Cogito ergo sum" — "I think therefore I am." Descartes has had a profound influence on mathematics and Western philosophy, but one of his major contributions to psychology actually came from being wrong.

Descartes believed in dualismdualismDescartes' view that the mind and body are two fundamentally different substances — the non-material mind controls the material body., the belief that the world could be divided into two main categories: the mental (mind) and the material (body). He believed the mind was not a physical substance, but controlled the body mechanistically. Descartes searched for the "seat of the soul" where the non-material mind interacted with the material body, believing he had found it in the pineal gland. As we'll see, Descartes was wrong about the function of the pineal gland, and most psychologists today believe he was wrong about the mind being separate from the body as well.

To remember Descartes and dualism, recall the unfortunate events following his death. Descartes died in Sweden in 1650, and 16 years later his corpse was exhumed for reburial in France. During the transfer, the French ambassador took Descartes' right index finger as a souvenir — and more memorably, Descartes' head was chopped off and his skull never made the journey back to France. It remained in private ownership until 1821, then placed at the Musée de l'Homme in Paris, separate from the rest of his body. How's that for a mnemonic for the separation of mind and body?

The fact that dualism feels right but is actually wrong shows us that our intuitions about psychology cannot always be trusted. Throughout this book we will find many examples of great minds gone wrong — a reminder that all minds, including our own, are always prone to error.

In contrast to Cartesian dualism, monismmonismThe view that mind and body are not separate — all thoughts, emotions, and behaviors arise from physiological processes. The guiding assumption of modern psychology. (from the Greek mono — "one") assumes that everything can be explained as having one physiological source. Monism holds that there is no mind or soul separate from the body — all thoughts, behaviors, emotions, and traits can be explained by our physiology. The widespread adoption of monism marked a shift to thinking of your mind as an experience derived from the physiological activity of your body. This is the guiding approach for modern psychology.

Jumping ahead a few centuries to the 1820s, we find another example where an error of the past has helped to inform our present understanding.

Franz Josef GallFranz Josef Gall19th-century anatomist (1758–1828) who developed phrenology — the claim that skull shape reveals mental faculties. Wrong in method but influential in directing attention to brain localization. (1758-1828) became curious about how brain structure might influence personality traits and abilities. Since brain scanning technologies were still more than a century away, Gall measured the skull, looking for patterns among bumps and dents that might map to mechanical ability, poetic talent, and other "fundamental faculties." This approach became known as phrenologyphrenologyThe now-discredited practice of inferring personality and mental abilities from the shape of the skull, associated with Franz Josef Gall..

This approach was eventually discarded, because skull shape is not a good indicator of underlying brain structure. But while phrenology came to be seen as the pseudo-science it was, its importance should not be ignored. It was one of the first approaches with the guiding assumption that specific brain areas perform specific tasks — known as localization of brain function. What Gall was really trying to show, despite his flawed approach, was that different areas of the brain do different things. This was a new idea at the time, but one that would eventually become a key concept underlying neurological research.

Throughout the 19th century and into the 20th century, attempts at answering psychological questions began to take on a more scientific approach. Advances in technology played a key role in this development.

Wilhelm WundtWilhelm WundtGerman psychologist (1832–1920) who established the first psychology laboratory in Leipzig in 1879, widely considered the founder of experimental psychology. (1832-1920) started a research laboratory in Leipzig with the goal of breaking consciousness down into measurable components. By understanding the structural components of consciousness, Wundt hoped to uncover how the mind operated as a whole. This approach, known as structuralismstructuralismThe first formal school of psychology, founded by Wundt and developed by Titchener, which aimed to break conscious experience into its basic elements using introspection., relied on introspectionintrospectionA method in which trained observers carefully report their own conscious experiences in response to stimuli — the main tool of structuralism. — carefully isolating the components of perception in order to understand consciousness. Edward TitchenerEdward TitchenerStudent of Wundt who brought structuralism to America, working for years to catalog the basic elements of consciousness via introspection. (1867-1927) took this to its extreme, creating a 44,000 term list of the essential elements of consciousness. Unfortunately, introspection's inevitably subjective responses meant results were difficult to verify.

FunctionalismfunctionalismAn early school of psychology, influenced by Darwin and associated with William James, that focused on the purpose and adaptive functions of mental processes rather than their structure.'s main proponent was William JamesWilliam JamesAmerican philosopher and psychologist (1842–1910), father of functionalism, who argued we must understand the purpose of mental processes — not just their structure. (1842-1910), a Harvard professor who believed we needed to understand the function of mental abilities. James was influenced by Charles DarwinCharles Darwin19th-century naturalist whose theory of natural selection profoundly influenced psychology, linking behavior and mental traits to biology and adaptation. (1809-1882), whose emphasis on the adaptive purpose of physical traits James applied to the mind.

Around the same time, Sigmund FreudSigmund FreudAustrian neurologist and founder of psychoanalysis (1856–1939), who emphasized unconscious drives, childhood experience, and unresolved conflict as the roots of mental illness. (1856-1939) in Vienna was developing the psychoanalytic approachpsychoanalytic approachFreud's framework emphasizing that behavior is driven by unconscious conflicts, repressed memories, and unresolved developmental stages.. Freud believed mental afflictions resulted from early childhood experiences continuing to operate unconsciouslyunconsciousThe part of the mind containing thoughts, memories, and desires outside conscious awareness that nonetheless influence behavior — Freud's central concept.. Though most of his specific theories have been rejected by modern researchers, the notion that unconscious processes influence behavior was a fundamental new insight.

From around 1920-1960, psychological research shifted toward observable behavior under the influence of behaviorismbehaviorismA school of psychology, associated with Watson and Skinner, that studies only observable behavior and rejects internal mental states as valid scientific data.. Led by John B. Watson (1878-1958) and later B.F. SkinnerB.F. SkinnerAmerican behaviorist (1904–1990) who developed operant conditioning, demonstrating how reinforcement and punishment shape voluntary behavior. (1904-1990), psychologists looked to quantify how the environment shapes behavior through rewards and punishments. Behaviorism gradually fell out of favor beginning in the 1960s.

The diversity of questions that psychology aims to answer necessarily requires a diversity of approaches. As psychology has grown to incorporate many sub-fields, greater specialization is required of researchers. No longer can one researcher be familiar with all areas of psychological inquiry. Most psychologists today fall into one of the following categories.

Cognitive psychologycognitive psychologyThe study of internal mental processes such as thinking, memory, perception, language, and problem-solving; emerged during the cognitive revolution of the 1960s. looks to understand how our thoughts influence our behavior. The way we think, use language, frame questions, and consider context can affect our decision-making, problem-solving, and emotional state.

Evolutionary psychologyevolutionary psychologyA subfield applying principles of natural selection to explain psychological traits and behaviors as adaptations shaped throughout human evolutionary history. continues Darwin's approach to uncover how psychological traits have been selected throughout human history. Developmental psychologydevelopmental psychologyThe study of how people change physically, cognitively, socially, and emotionally across the lifespan — from birth to death. seeks to understand progression of biological and cognitive changes from birth all the way to old age. NeuroscienceneuroscienceThe study of the structure and function of the nervous system and brain as they relate to behavior and mental processes. looks to understand links between brain activity, thought, and behavior — an area with major recent advances thanks to brain imaging technology.

Social psychologysocial psychologyThe study of how individuals' thoughts, feelings, and behaviors are influenced by the actual or imagined presence of others. looks to understand the complexity of our interpersonal interactions, from the perils of stereotypes and aggression to the heart-warming moments of empathy and altruism. Industrial-organizational psychologyindustrial-organizational psychologyThe application of psychological principles to workplace settings, including employee selection, motivation, leadership, and organizational behavior. looks to understand how to improve worker morale, productivity, and well-being in the workplace.

Cultural psychologycultural psychologyThe study of how culture shapes thoughts, emotions, and behaviors — showing the ways culture defines us as well as the ways we all share one human experience. has looked to tease apart the influences of culture on our thoughts, emotions, and behaviors. And the sub-field most people think of when they hear the word psychology is abnormal psychologyabnormal psychologyThe study of psychological disorders — their causes, diagnosis, and treatment. Incorporates biological, psychological, and social approaches. — the study of mental illness. This field brings together psychiatristspsychiatristA medical doctor (MD/DO) who specializes in mental health and is licensed to prescribe medications., clinical psychologistsclinical psychologyA branch of psychology focused on assessing, diagnosing, and treating psychological disorders through psychotherapy., and counseling psychologistscounseling psychologyA branch of psychology that helps people with mild to moderate difficulties, including relationship issues, career concerns, and life transitions..

If this dizzying array of approaches is stressing you out, remember to step back and consider the purpose for each. Each individual approach is just another way of attempting to answer a psychological question. Should we focus our research on possible genes responsible or childhood experiences? On hormones or cultural influences? Remember that each focus temporarily ignores the others in order to simplify very complex questions. It's easier to ask how genes might relate to depression than to try to understand everything about depression all at once.

These different approaches are really just ways to structure our thinking, and we need to be careful not to become trapped in a particular structure. Staying isolated in one of these "boxes" can be useful for making progress and staying focused when dealing with complex interactions, but we need to remember that each box is just part of the picture. We should remember that a strictly biological explanation may be ignoring crucial cognitive factors, or that a psychoanalytic approach may be biased in its cultural understanding of illness.

As I described in the introduction, psychology is inherently multidisciplinary. It cannot be reduced to a single approach. While each individual approach is helpful, we want to avoid seeing any single approach as the approach. So while you may be inclined to prefer one approach, be careful not to be dismissive of the others, as they just may hold the puzzle pieces you're missing.