What exactly is mental illness? What do we mean when we say that someone is either mentally ill or is experiencing a mental illness? And who decides what a mental illness is in the first place?

In order to understand the concept of mental illness, it is necessary to understand the broader notion of illness generally. Human beings have had the need–and the ability–to distinguish health from illness for millennia. The capacity to recognize states of illness–both physical and mental–undoubtedly plays a role in the survival of our species. This ability helped human populations survive epidemics of communicable diseases, such as the Black Plague and Tuberculosis. 

Nevertheless, what any society at any particular point in time deems an illness is mutable. Central to the idea of illness, conceptually, for most of us, is that it is the absence of health. Indeed, the Oxford English Dictionary (OED) suggests that the prevailing modern sense of the term ill is “out of health, sick, indisposed, not well.”

So, what then is health?

As its first entry, the OED defines health as “soundness of body; that condition in which its functions are duly and efficiently discharged.” Modern English health is derived from the Old English hǽlþ, which could be defined as “wholeness, a being whole, sound or well.” Hǽlþ itself is derived from from Proto-Germanic hailitho, which is derived from Proto Indo-European kailo, meaning “whole, uninjured, of good omen”

Moreover, the OED proposes a more archaic concept of health in its fourth entry: “Spiritual, moral, or mental soundness or well-being; salvation.”

Etymologically speaking, health appears to connote various root conceptual elements including wholeness and soundness of body and mind, efficient functioning, lack of injury and, curiously, the moral element of goodness.  (Meanwhile, the root word of illness–ill–is derived from the Old Norse illr, meaning “evil or bad.”)

Unsurprisingly perhaps, in view of the fact that I’m a physician by training and education and have a background in the biological sciences, my own conceptualization of health–and the core conceptual elements embedded in the term–deviates some from the commonly understood notion of health. For me, health encompasses three distinct ideas. 

The first is that it is a state of being of something that signifies optimal functioning of that something, as in a “healthy” economy, a “healthy” democracy or a “healthy” relationship. Health, when specifically applied to biological organisms could therefore be understood to indicate a state of being of an organism that results in optimal functioning of that organism. In this vein, component parts of an organism can also be described in terms of health and illness. A healthy kidney, for example, functions optimally to balance electrolytes in our blood, maintain acid-base balance, and eliminate certain types of waste products. A healthy heart pumps blood efficiently to feed the organs of the body. A healthy gut functions optimally to absorb nutrients in the food that we eat and eliminate the non-nutritive components of food. And so on and so forth.

When we examine the life cycles of all organisms on Earth, we can observe that biological organisms, when functioning optimally, are programmed to survive, adapt to their environment, grow and reproduce–thus ensuring that the biological variables responsible for the organism’s success are passed onto its progeny.

Be that as it may, another undeniable aspect of life on Earth is that all biological organisms on our planet eventually succumb to age and cease to function. One could argue that the parameters of life on Earth have been optimized such that the resources needed by young organisms to survive, grow and reproduce are safeguarded by the eventual demise of the aging organisms that gave rise to them in the first place. Since life first evolved on Earth, this same process has resulted in countless cycles of birth and death.

If optimal functioning of an organism is one fundamental aspect of health, a second elemental concept in the larger construct of health, in my view, is the absence of premature demise. Should death come to us earlier than anticipated and not result from some chance factor outside of ourselves, it is reasonable to conclude that our health was somehow compromised. (Indeed, any number of illnesses can result in premature death in humans, from exceedingly rare conditions such as xeroderma pigmentosum to relatively rare conditions such as Down’s syndrome and cystic fibrosis to common diseases such as heart disease and cancer.)

The third foundational component of my definition of health is the absence of suffering. For millennia, human societies have conjoined the idea of suffering to the concept of ill health. Suffering in human beings reveals itself in varied forms: physical pain, emotional pain, and other types of distress related to the myriad symptoms that we are capable of experiencing, from profound fatigue to persistent nausea to lightheadedness or shortness of breath. Suffering, according to the OED, is “the bearing or undergoing of pain, distress, or tribulation.”

Personally, I believe that suffering, itself, impacts the functioning of an organism. Humans who suffer beyond the everyday suffering inherent to life cannot function as optimally as those who do not. Simply put, suffering depletes the organism of mental and physical resources because it is a type of encumbrance with an energetic cost to the organism. When we humans suffer, a certain amount of our mental “bandwidth” is taken up by the experience of suffering–whether physically-based or mental–and in the management of the suffering. Pain, for example, is an extra signal generated in the central nervous system that cuts into our conscious experience on a moment to moment basis and, unlike other sensory experiences (such as simply seeing what you do now), has a noxious element. The experience of pain, whether mental or physical, degrades the cognitive capacity of our brains, resulting in impaired attention and productivity. Moreover, processes that either generate the experience of pain or arise from it, such as persistent negative ruminations in depression, can also directly diminish our cognitive capacity, motivation and ability to get things done.

To summarize, then, illness,–as viewed as the absence of health–subsumes three foundational concepts:

1. Decreased functioning of the organism
2. Premature death
3. Excessive suffering beyond the normal suffering of day-to-day life

Mental illnesses, therefore, are illnesses of the mind that result in decreased functioning, premature death or excessive suffering–or any combination of these features. Since it is generally understood now that our minds are the experience of reality generated by our brains, for all intents and purposes, mental illnesses are brain-based illnesses that affect our thoughts, our emotions, our behaviors and other mental phenomena (e.g. attention, executive functioning, processing speed, etc.).

In the United States, the most common mental illnesses are anxiety disorders, alcohol and substance use disorders, mood disorders, Attention-Deficit/Hyperactivity Disorder and personality disorders. In 2022, it is estimated that anywhere between 1 in 5 to 1 in 4 adults in the United States experience a mental illness on an annual basis.

The foremost classification system of mental disorders at present is a nosological compendium known as the Diagnostic and Statistical Manual of Mental Disorders 5th Edition Text Revision (DSM-5-TR), published by the American Psychiatric Association (APA). It defines a mental disorder as follows:

…a syndrome characterized by clinically significant disturbance in an individual’s cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or development processes underlying mental functioning. Mental disorders are usually associated with significant distress or disability in social, occupational, or other important activities. 

If you accept the definition of mental disorder proposed by the APA, you would understand it to be a group of illnesses that affect one’s thoughts and thinking, emotions (the conscious experience of which we might call feelings or moods) and behaviors. 

My physicalist position is that all mental phenomena–cognitions, emotions, sensory perceptions, memory, decisions–have their genesis in the functional activity of the brain. The current scientific understanding of mind (our Modern English word ultimately being derived from the Old English mynd/gemynd (memory, remembrance), which itself is derived from Proto Germanic *ga-mundiz (remembrance, memory), derived from  Proto-Indo-European *ménos (mind, thought)) is that it is one byproduct of the ever-changing electrical activity of our brains. Fundamentally, our brains are electrical organs, each comprising approximately 100 billion neurons–specialized cells with a unique morphology that communicate with other neurons, as well as other types of cells, such as muscle cells and endocrine cells.

 Moreover, the brain is a highly organized structure with different regions generating different aspects of our lived experiences (sensory experiences, memories, thoughts, feelings) and behaviors. At this moment in time, understanding the functioning of our brains through technologies such as functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) affords us some of the most powerful ways to understand the biological basis of mental illness. At the resolution of structural imaging technologies that are currently available (e.g. computed tomography and magnetic resonance imaging), the vast majority of mental disorders do not appear to produce—or stem from—grossly visible structural changes of the brain. In other words, examining a standard magnetic resonance imaging (MRI) of the brain of a depressed individual does not reveal anything materially aberrant relative to an MRI of the brain of a “healthy control”. 

Nonetheless, it is essential to understand the basic tenet that discrete regions of the brain are involved in the genesis of the array of conscious mental phenomena we experience, such as thoughts and emotions. Other structures are involved in the initiation of everyday activities from voluntary movement to planning. Yet other structures are involved in the regulation of vital bodily functions such as heart rate, blood pressure, respiratory rate and the initiation and maintenance of sleep. This rudimentary understanding  allows us to hypothesize that symptoms of mental illness are the result of dysfunction within and across discrete regions (neuronal populations) of the brain involved in the regulation of thoughts, emotions and certain behaviors.

At a gross anatomical level, our brains can be divided into 3 main regions: the cerebrum, the cerebellum and the brain stem. 

The cerebrum and cerebellum are also divided into two hemispheres. The cerebrum, furthermore, can be divided into 4 lobes: frontal, parietal, temporal and occipital. 

A variety of lesioning and stimulation studies over many decades have revealed that neural activity in each of these regions corresponds to different functions. 

For example, a particular area of the frontal lobe known as Broca’s area (named after the 19th century French physician who deduced its function) is responsible for speech production, including planning the motoric elements of speech. It is situated in the inferior (lower) part of the frontal lobe, just anterior (in front of) the premotor cortex.

Strokes (cerebrovascular accidents) involving this region of the brain commonly result in expressive aphasia, a condition in which speech comprehension is preserved, but speech production is impaired. 

Another region of the frontal lobe, referred to as the primary motor cortex, is responsible for directed movement of skeletal muscles.

Our movements are also fine-tuned by neural activity in the cerebellum. Planning of movement is directed by neural activity within the prefrontal cortex and the premotor cortex.

The term “neural circuit” is used to describe a collection of neurons that communicate with one another and that correspond with a particular function. Just as activity within certain neural circuits produces our sensory experiences, activity within other neural circuits results in our experience of thoughts and emotions and in the production of behaviors–the chief domains of mental disorders.

One arm of modern psychiatry posits that mental illnesses result from dysfunction within neural circuits that correspond to various aspects of our cognitions, emotions and behaviors. Sometimes, the dysfunction arises from observable structural changes in the brain–as in the neurodegenerative conditions of Parkinson’s Disease and Alzheimer’s Disease–or physical trauma, such as a traumatic brain injury (TBI). Sometimes, microstructural changes occur early in life and result in abnormal brain development, as in Autism. Sometimes, the microstructural changes result from excessive synaptic “pruning”–a neurodevelopmental process by which excessive or redundant synaptic connections are eliminated–in adolescence or early adulthood, as in the Feinberg hypothesis of Schizophrenia. 

Other times, microstructural alterations and functional changes in the behavior of neural circuits are the consequence of more obvious environmental factors such as experiencing or witnessing life-threatening trauma, as in Post-Traumatic Stress Disorder. Cumulative adverse childhood experiences–or, ACEs, as they are referred to in the professional literature–also result in functional changes in neural circuits, leading to a higher vulnerability to mood and anxiety disorders and other mental disorders. (Both childhood trauma and PTSD also appear to cause microstructural changes in the brain.)

The neural circuit basis of many mental disorders is being fleshed out presently, using functional–as opposed to purely structural–neuroimaging techniques such as fMRI. Data from these studies, which compare and contrast functional imaging of the brains of individuals with mental disorders with those of “healthy controls”, allow researchers to come up with diagrams such as this one, which shows discrete brain regions and neural circuits involved the symptoms of depression:

Available evidence also demonstrates that electrical manipulation of these neural circuits, using techniques such as transcranial magnetic stimulation (TMS) and closed loop deep brain stimulation, alleviates symptoms of depression.

Our brain is an ever-changing organ that responds to our environments and our experiences. The brain’s ability to change itself is referred to as neuroplasticity. Positive experiences can have a healing effect on our mental health, while negative experiences can damage our mental health, sometimes profoundly so. Indeed, experiential therapies such as psychotherapy (“talk therapy”), which form one cornerstone of the evidence-based treatments for mental disorders, work because they alter neural circuit dysfunction in the brains of affected individuals, thereby mitigating symptoms.

Now that we have a more nuanced understanding of mental illness as a concept (i.e. brain-based diseases that affect one’s cognitions, emotions and behaviors and that result in impaired  functioning, premature death and excessive suffering), we can ask ourselves: So, who decides what exactly constitutes a mental illness?

DSM-5-TR, and the editions of the DSM before it, attempt to categorize mental disorders into discrete categories such as Neurodevelopmental Disorders, Schizophrenia Spectrum and Other Psychotic Disorders, Bipolar and Related Disorders, Personality Disorders, Sleep-Wake Disorders, etc. To the casual observer, it would appear as though there were some rhyme or reason to this categorization system. A disorder of eating and feeding behavior, such as Bulimia Nervosa, must surely be something different from a psychotic disorder, such as Schizophrenia, in which someone might hear voices and have paranoid delusions. 

However, it must not be forgotten that, at a larger level, it is a classification system that was developed by one organization, the APA, in 1952. Moreover, the APA, being the largest organization of psychiatrists in the United States, is a societally sanctioned organization with significant clout in the American medical system and one that spent nearly one million dollars in lobbying efforts in 2018. In one sense, therefore, mental disorders—their categorization and diagnosis–are societally sanctioned disorders of the mind, a point that is not lost on those who view psychiatry, as a medical discipline, as uniquely oppressive. For reasons that are too long to explore herein, the sum of our conscious and unconscious mental phenomena–our minds–are uniquely guarded in a way that the functions of other organ systems simply are not. Because who we are is intimately related to our mental experience of ourselves, it is often more difficult to distance ourselves from diseases that affect our minds than, say, our hearts. People don’t seem to struggle with the notion that they have an arrhythmia (an abnormal heart rhythm or rate) in the way that they do when told that they have a Bipolar Disorder.

There were other attempts to categorize mental disorders prior to the DSM, of course, including the International Classification of Diseases 6^th Edition (ICD-6), in 1949. In ancient Greece, Hippocrates, the renowned physician, described the following mental disorders: Mania, Melancholy, Phrenitis, Insanity, Disobedience, Paranoia, Panic, Epilepsy, and Hysteria. However, his explanatory model for these disorders was based upon the unscientific humoral theory which posited that disorders, including mental disorders, developed as a consequence of the  imbalance in the four “humors” of the body: blood, phlegm, black bile and yellow bile.

Emil Kraeplin, the German psychiatrist, is generally considered the father of modern psychiatry. Kraeplin developed a classification system of mental disorders that distinguished what is now known as Bipolar Disorder from another related disorder,  Schizophrenia.

Physicians, such as Hippocrates and Kraeplin, over many centuries have attempted to identify specific diseases and disorders based upon the recognition of discrete symptoms (things that patients describe) and signs (things that physicians observe) that tend to go together (a syndrome). Categorizing physical and mental disorders as syndromes allows physicians and other health care practitioners and scientists, among other things, to engage in the scientific study of these conditions and to use a common language. The scientific study of particular disorders, in turn, allows us to test hypotheses related to that disorder and ultimately determine the prognosis of patients with those disorders as well as to identify effective treatments.

Fortunately, the APA and the DSM, in delineating the diagnostic criteria for the various mental disorders, have attempted to create scientifically valid constructs that have some (although certainly not perfect) reliability in diagnosis. 

A mental disorder, in summary, is many things. First and foremost, perhaps, it is an illness–a condition that leads to suboptimal functioning and excessive suffering and has the potential to shorten the lifespan. More specifically, it is an illness of the mind that affects one’s cognitions, emotions and behaviors.  The mind itself is the sum of mental phenomena generated by the activity of our brains. As such, mental disorders are disorders of the brain that give rise to the experience of certain symptoms (e.g. depressed mood, inability to have pleasure, low energy)  and certain observable signs  (e.g affective flattening, psychomotor slowing, reduced prosody of speech). The natural grouping of symptoms and signs that frequently occur together constitutes a syndrome. This grouping is formalized and codified by the aforementioned group of physicians who then have the ability to render diagnoses upon individuals in the society they serve. This formalization and codification of mental illnesses allows scientists an easy way to more effectively study them and for members of society to have a shared language of such phenomena. 

Finally, because we so closely identify our “self” as either the sum of our mental phenomena or that sum plus the concept of a corporeal self that occupies space in the physical world, the diagnosis of a mental illness frequently touches upon the notion of a disturbed, defective or damaged self–and not simply the organ that gives rise to the self. As an almost inevitable consequence, having a mental illness, living with one or receiving the diagnosis of one frequently goes hand in hand with the experience of unremitting shame. To the extent that human beings, by nature, are a species prone to judgment, it is unlikely that those afflicted with mental illness will ever be truly free to divulge their experiences and be fully free to express themselves–because judgment–social judgment– itself, is integral to the human experience and human survival.