Essentialist thinking is the belief that familiar categories—dogs and cats, space and time, emotions and thoughts—each have an underlying essence that makes them what they are. This belief is a key barrier to scientific understanding and progress. In pre-Darwinian biology, for example, scholars believed each species had an underlying essence or physical type, and variation was considered error. Darwin challenged this essentialist view, observing that a species is a conceptual category containing a population of varied individuals, not erroneous variations on one ideal individual. Even as Darwin's ideas became accepted, essentialism held fast, as biologists declared that genes are the essence of all living things, fully accounting for Darwin's variation. Nowadays we know that gene expression is regulated by the environment, a discovery that—after much debate—prompted a paradigm shift in biology.
In physics, before Einstein, scientists thought of space and time as separate physical quantities. Einstein refuted that distinction, unifying space and time and showing that they are relative to the perceiver. Even so, essentialist thinking is still seen every time an undergraduate asks, "If the universe is expanding, what is it expanding into?"
In my field of psychology, essentialist thought still runs rampant. Plenty of psychologists, for example, define emotions as behaviors (e.g., a rat freezes in fear, or attacks in anger), each triggered automatically by its own circuit, so that the circuit for the behavior (freezing, attacking) is the circuit for the emotion (fear, anger). When other scientists showed that, in fact, rats have varied behaviors in fear-evoking situations—sometimes freezing, but other times running away or even attacking—this inconsistency was "solved" by redefining fear to have multiple types, each with its own essence. This technique of creating ever finer categories, each with its own biological essence, is considered scientific progress, rather than abandoning essentialism as Darwin and Einstein did. Fortunately, other approaches to emotion have arisen that do not require essences. Psychological construction, for example, considers an emotion like fear or anger to be a category with diverse instances just as Darwin did with species.
Essentialism can also be seen in studies that scan the human brain, trying to locate the brain tissue that is dedicated to each emotion. At first, scientists assumed that each emotion could be localized to a specific brain region (e.g., fear occurs in the amygdala), but they found that each region is active for a variety of emotions, more than one would expect by chance. Since then, scientists have been searching for the brain essence of each emotion in dedicated brain networks, and in probabilistic patterns across the brain, always with the assumption that each emotion has an essence to be found, rather than abandoning essentialism.
The fact that different brain regions and networks show increased activity during different emotions is not a problem just for emotion research. They also show increased activation during other mental activities such as cognitions and perceptions, and have been implicated in mental illnesses from depression to schizophrenia to autism. This lack of specificity has led to claims (in news stories, blogs, and popular books) that we have learned nothing from brain imaging experiments. This seeming failure is actually a success. The data are screaming out that essentialism is wrong: individual brain regions, circuits, networks and even neurons are not single-purpose. The data are pointing to a new model of how the brain constructs the mind. Scientists understand data through the lens of their assumptions, however. Until these assumptions change, scientific progress will be limited.
Some topics in psychology have advanced beyond essentialist views. Memory, for example, was once thought to be a single process, and later was split into distinct subtypes like semantic memory and episodic memory. Memories are now considered to be constructed within the brain's functional architecture and not to reside in specific brain tissue. One hopes that other areas of psychology and neuroscience will soon follow suit. For example, cognition and emotion are still considered separate processes in the mind and brain, but there is growing evidence that the brain does not respect this division. This means every psychological theory in which emotions and cognitions battle each other, or in which cognitions regulate emotions, is wrong.
Ridding science of essentialism is easier said than done. Consider the simplicity of this essentialist statement from the past: "Gene X causes cancer." It sounds plausible and takes little effort to understand. Compare this to a more recent explanation: "A given individual in a given situation, who interprets that situation as stressful, may experience a change in his sympathetic nervous system that encourages certain genes to be expressed, making him vulnerable to cancer." The latter explanation is more complicated, but more realistic. Most natural phenomena do not have a single root cause. Sciences that are still steeped in essentialism need a better model of cause and effect, new experimental methods, and new statistical procedures to counter essentialist thinking.
This discussion is more than a bunch of metaphysical musings. Adherence to essentialism has serious, practical impacts on national security, the legal system, treatment of mental illness, the toxic effects of stress on physical illness... the list goes on. Essentialism leads to simplistic "single cause" thinking when the world is a complex place. Research suggests that children are born essentialists (what irony!) and must learn to overcome it. It's time for all scientists to overcome it as well.