"I was much struck how entirely vague and arbitrary is the distinction between species and varieties." Darwin (1859)
For centuries, there was one way to bring order to the vastness of biological diversity—Linnaean classification. In the 18th century, Carl Linnaeus devised a method for dividing species based upon their description—do they look the same, do they behave the same? With Linnaean classification, you could divide the natural world into discrete kinds—you could count them, saying confidently "there are two species of elephants" or "there are four kinds of bears." Some psychologists seek to bring the same order to the mind, claiming that "there are six emotions," "there are five types of personality," or "there are three moral concerns." These psychologists are inspired by the precision, order and neatness of Linnaeus' ideas—the only problem is that Linnaeus was wrong.
Linnaeus lived about a hundred years before Darwin introduced the theory of evolution, and long believed that species were fixed and unchangeable. His religious roots led him to see species as a product of divine providence and his job was simply to catalog these distinct kinds, once writing "God created, Linnaeus ordered." If God created a certain number of distinct species, cataloging and counting them made sense. It was meaningful to ask "how many salamanders did God create?"
Evolution, however, destroyed the sanctity of species. Species were not created whole from The Beginning, but instead emerged over time through the repetition of a simple algorithm: heredity, mutation and selection. Evolution showed that a dizzying diversity of life—from viruses, to cacti, to humans—were explained through a basic set of common processes expressed in different environments. This common process means that lines between species are more in the mind of humans than in nature, with many intermediate animals (e.g., lungfish) and hybrids (e.g., ligers) that defy easy categorization. Moreover, in geological time, these divisions are even more arbitrary, with species diverging and converging as continents separate and collide.
Biology has all but realized that species are not reflections of eternal Divine Order, but simply a useful way to intuitively organize the world. Unfortunately, psychology lags behind. Many psychologists believe that the mental world is fixed and countable, that the appearance of mental states reflects a deeper essence. Introductory psychology textbooks contain numbered lists of psychological species—5 kinds of human needs, 6 basic emotions, 3 moral concerns, 3 kinds of love, 3 parts of the mind—with these lists depending primarily upon the intuitions of those who are doing the counting.
Like Linnaeus in the 18th century, these intuitive taxonomies were once the best we could do because psychology lacked an understanding of basic psychological process. However, social cognition and neuroscience has revealed these processes, and found that diverse mental experiences—from emotion to morality to motivation—are combinations of more basic affective and cognitive processes. This research suggests that psychological states are not firmly demarcated "things" with enduring essences, but are instead fuzzy constructs that emerge from common psychological processes expressed across different environments.
Just as evolution can create infinite species by expressing a common process in specific environments, so too can the mind create infinite mental species. One can no sooner count emotions or moral concerns than snowflakes or colors. To be sure, there are descriptive similarities and differences across instances, but any groupings are arbitrary and rest heavily on the intuition of researchers. This is why scientists can never agree on the fundamental number of anything; one scientist may divide a mental experience into 3, another 4, and another 5.
It is time for psychology to abandon the enterprise of numbering nature, and recognize that psychological species are neither distinct nor real. Biology has long recognized the arbitrary and constructed natured of species; why are we more than 200 years behind? The likely answer is that people—even including psychologists and philosophers—believe that intuitions, as products of the mind, are accurate reflections of its structure. Unfortunately, decades of research demonstrates the flaws of intuitive realism, revealing that intuitions about the mind are poor guides to underlying psychological processes.
Psychologists must move from counting to combining. Counting is simply describing the world; one psychologist's intuitive ordering of mental states in one culture, at one time. Combining seeks to find basic psychological elements and discover how they interact to create the mental world. In biology, counting asks "how many salamanders are there?" whereas combining asks "what processes lead to salamander diversity." Counting is bound to a specific environment and time, whereas combining recognizes these factors as processes themselves. Psychology must follow biology and move from numbering individual species to exploring underlying systems.
This process has already begun. Thomas Insel, the head of NIMH has prioritized systems over species in psychopathology research. He rejects the utility of the DSM, suggesting that intuitive taxonomies obscure underling process of psychopathology, and impedes the discovery of treatments. NIMH funds proposals that examine the underlying affective, conceptual and neurological systems, which may explain why the "distinct" disorders of depression and anxiety are so often comorbid, and why serotonin reuptake inhibitors (SSRIs) appear to help diverse disorders. Psychopathology does not easily fit into categories, and neither do other psychological phenomena.
Of course, we shouldn't throw out the baby with the bathwater. It is still necessary to catalog the natural world to allow meaningful discussion. Even in biology, where the power of the process of evolution is undisputed, most acknowledge the utility of Linnaeus' system and continue to use the names he provided years ago. But he key is not to confuse human constructions with natural order; what is useful to humans is not necessarily true of nature. Intuitive taxonomies are a necessary first step in psychological science but even Linnaeus—as he learned more about the world—recognized the arbitrariness of his system and the species he labeled. It is time for psychology to recognize this fact as well, and leave behind Linnaeus and the 18th century.