In most realms of human endeavor, no one person ever stands out as clearly greater than the other greats in that field. It would be impossible to say who was the greatest philosopher, or physicist, or artist, or baseball player, or movie star. Because so many are great, no one person ever shines brightest.

Psychology is an exception -- Charles Darwin does tower over this field.

Born 204 years ago on February 12, Darwin was deeply interested in comparing human and animal psychology as a test case and fascinating application of evolutionary theory.

His careful naturalistic observation, combined with deep psychological insight, completely changed how we understand human nature.

Darwin's most fundamental conclusions:

•We are animals -- just part of the grand tableau of creation, not its purpose.

•Our instincts, emotions, and intellect evolved from a common primate ancestor -- just as completely as did our bodily form.

•We can understand ourselves best by studying the psychological, as well as the physical, steps in that evolution.

•Our psychology is the product of the natural and sexual selection of reproductively adaptive chance variants -- it was not preplanned or inspired by divine intervention.

•The mind and its consciousness are a product of brain functioning in a way that is not essentially different than digestion is a function of the gut.

•Psychology can be studied using the standard experimental and observational tools of science.

•People all over the world, despite differences in their current customs, are brothers and sisters within one human species, sharing the same basic emotions and intellectual endowment.

•The child is father to the man -- we can learn about the psychology of the individual and evolution of the species by carefully studying the maturation of behaviors in infants and children.

•Instincts are not completely fixed but interact with the environment.

•Unconscious forces play a large role in influencing are our behaviors.

And Darwin also established novel methods of psychological study that have since become standards in the field:

•His statement that we can learn more about ourselves by studying baboons than by reading the great philosophers created the field of evolutionary psychology and provided the opportunity for deep insights into human motivation and behavior.

•Darwin's Biographical Sketch of an Infant detailing his minute, naturalistic observations of the day to day emotional, intellectual, interpersonal, and moral development of his eldest son created the field of child development.

•Darwin's method of studying emotions and facial expressions using photographs he commissioned for this purpose is still an enormously fruitful research tool.

•Darwin conducted the first survey in psychology -- a written instrument gathering information from scientists and missionaries to show the universality of human emotions all around the world.

•Darwin also pioneered in subjective introspection, including the self analysis of his own dreams.

It is startling that Darwin had made most of his major psychological discoveries before his 30th birthday and even before he realized that natural selection is the mechanism of evolution.

He kept these findings in a drawer for 35 years before finally publishing them -- partly because he was a meticulous collector of facts before presenting theories; partly because he realized that the world was not ready for such a materialistic view of man; and partly because he didn't like confrontation with critics.

Newton modestly described himself as a dwarf sitting on the shoulders of the preceding giants. In psychology, it was Freud sitting on Darwin's shoulders -- ingeniously applying Darwin's evolutionary insights to the wide world of psychological symptoms, dreams, myth, art, anthropology, and the vicissitudes of everyday life.

Freud was 26 when Darwin died and they never met, but almost all of his mentors were enthusiastic Darwinists.

In Freud's day, psychologists and neuroscientists all spoke 'Darwin' even if they didn't always realize it; just as today we all unconsciously speak some dialect of 'Freud'.

Freud's biographer Ernest Jones called Freud 'the Darwin of the mind.' In fact, Darwin was the Darwin of the mind, with Freud as his great popularizer.

The most important step forward in our understanding human psychology was the realization that much of our mental life is automatic, unconscious, and outside the control of our reason or will.

Lots of philosophers, scientists, and writers helped explore this realm of the unconscious, before and after Darwin.

But Darwin was by far the most important because, by connecting the mind of man with our primate past, he was able to fill in so many of the blanks -- explaining why we do what we do and feel what we feel.

Psychology since has been an enormously exciting, but mostly derivative, elaboration of the grand model intuited by Darwin 175 years ago. We now have the wonderful new tools of neuro and cognitive science to explore how brain makes mind -- but our modern conception of human nature was all there in Darwin's notebooks.

Allen Frances is a professor emeritus at Duke University and was the chairman of the DSM-IV task force.